Create Scala 2 veneer for SourceInfo

The SourceInfo hierarchy is sealed, and very basic, so most of the code
is just moved to src/main/scala-2.

core/src/main/scala-2/chisel3/experimental/SourceInfo.scala

@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: Apache-2.0
+
+// This file contains macros for adding source locators at the point of invocation.
+//
+// This is not part of coreMacros to disallow this macro from being implicitly invoked in Chisel
+// frontend (and generating source locators in Chisel core), which is almost certainly a bug.
+//
+// Note: While these functions and definitions are not private (macros can't be
+// private), these are NOT meant to be part of the public API (yet) and no
+// forward compatibility guarantees are made.
+// A future revision may stabilize the source locator API to allow library
+// writers to append source locator information at the point of a library
+// function invocation.
+
+package chisel3.experimental
+
+import scala.language.experimental.macros
+import scala.reflect.macros.blackbox.Context
+import chisel3.internal.sourceinfo.SourceInfoMacro
+
+/** Abstract base class for generalized source information.
+  */
+sealed trait SourceInfo {
+
+  /** A prettier toString
+    *
+    * Make a useful message if SourceInfo is available, nothing otherwise
+    */
+  def makeMessage(f: String => String = x => x): String
+
+  /** The filename for the originating source file, if known */
+  def filenameOption: Option[String]
+}
+
+sealed trait NoSourceInfo extends SourceInfo {
+  def makeMessage(f: String => String = x => x): String = ""
+  def filenameOption: Option[String] = None
+}
+
+/** For when source info can't be generated because of a technical limitation, like for Reg because
+  * Scala macros don't support named or default arguments.
+  */
+case object UnlocatableSourceInfo extends NoSourceInfo
+
+/** For when source info isn't generated because the function is deprecated and we're lazy.
+  */
+case object DeprecatedSourceInfo extends NoSourceInfo
+
+/** For FIRRTL lines from a Scala source line.
+  *
+  * @note A column == 0 indicates no column
+  */
+case class SourceLine(filename: String, line: Int, col: Int) extends SourceInfo with SourceLineImpl {
+  def makeMessage(f: String => String = x => x): String = _makeMessageImpl(f)
+}
+
+object SourceInfo extends ObjectSourceInfoImpl {
+  implicit def materialize: SourceInfo = macro SourceInfoMacro.generate_source_info
+}

core/src/main/scala/chisel3/experimental/SourceInfoImpl.scala

@@ -1,57 +1,14 @@
 // SPDX-License-Identifier: Apache-2.0
 
-// This file contains macros for adding source locators at the point of invocation.
-//
-// This is not part of coreMacros to disallow this macro from being implicitly invoked in Chisel
-// frontend (and generating source locators in Chisel core), which is almost certainly a bug.
-//
-// Note: While these functions and definitions are not private (macros can't be
-// private), these are NOT meant to be part of the public API (yet) and no
-// forward compatibility guarantees are made.
-// A future revision may stabilize the source locator API to allow library
-// writers to append source locator information at the point of a library
-// function invocation.
-
 package chisel3.experimental
 
-import scala.language.experimental.macros
-import scala.reflect.macros.blackbox.Context
-import chisel3.internal.sourceinfo.SourceInfoMacro
-
-/** Abstract base class for generalized source information.
-  */
-sealed trait SourceInfo {
-
-  /** A prettier toString
-    *
-    * Make a useful message if SourceInfo is available, nothing otherwise
-    */
-  def makeMessage(f: String => String = x => x): String
-
-  /** The filename for the originating source file, if known */
-  def filenameOption: Option[String]
-}
-
-sealed trait NoSourceInfo extends SourceInfo {
-  def makeMessage(f: String => String = x => x): String = ""
-  def filenameOption: Option[String] = None
-}
-
-/** For when source info can't be generated because of a technical limitation, like for Reg because
-  * Scala macros don't support named or default arguments.
-  */
-case object UnlocatableSourceInfo extends NoSourceInfo
+private[chisel3] trait SourceLineImpl {
+  def filename: String
+  def line:     Int
+  def col:      Int
 
-/** For when source info isn't generated because the function is deprecated and we're lazy.
-  */
-case object DeprecatedSourceInfo extends NoSourceInfo
+  protected def _makeMessageImpl(f: String => String = x => x): String = f(s"@[${this.prettyPrint}]")
 
-/** For FIRRTL lines from a Scala source line.
-  *
-  * @note A column == 0 indicates no column
-  */
-case class SourceLine(filename: String, line: Int, col: Int) extends SourceInfo {
-  def makeMessage(f: String => String = x => x): String = f(s"@[${this.prettyPrint}]")
   def filenameOption: Option[String] = Some(filename)
 
   private def prettyPrint: String = {
@@ -64,8 +21,7 @@ case class SourceLine(filename: String, line: Int, col: Int) extends SourceInfo
   }
 }
 
-object SourceInfo {
-  implicit def materialize: SourceInfo = macro SourceInfoMacro.generate_source_info
+private[chisel3] trait ObjectSourceInfoImpl {
 
   /** Returns the best guess at the first stack frame that belongs to user code.
     */