Keep track of source files created for macro expansions and such.

Introduce a new source file kind to describe source files for macro
expansions, and include the macro expression that they expand. This
establishes a "parent" relationship

Also track every kind of auxiliary source file---whether for macro
expansions or other reasons---that is introduced into a module, adding
an operation that allows us to find the source file that contains a
given source location.

lib/Sema/CodeSynthesisDistributedActor.cpp

@@ -788,6 +788,7 @@ addDistributedActorCodableConformance(
 
         case SourceFileKind::Library:
         case SourceFileKind::Main:
+        case SourceFileKind::MacroExpansion:
         case SourceFileKind::SIL:
           break;
         }