[Macros] Top-level freestanding macros (#63553)

Allow freestanding macros to be used at top-level.
- Parse top-level `#…` as `MacroExpansionDecl` when we are not in scripting mode.
- Add macro expansion decls to the source lookup cache with name-driven lazy expansion. Not supporting arbitrary name yet.
- Experimental support for script mode and brace-level declaration macro expansions: When type-checking a `MacroExpansionExpr`, assign it a substitute `MacroExpansionDecl` if the macro reference resolves to a declaration macro. This doesn’t work quite fully yet and will be enabled in a future fix.

lib/Serialization/Serialization.cpp

@@ -2246,7 +2246,7 @@ static uint8_t getRawStableMacroRole(swift::MacroRole context) {
   }
 #undef CASE
   llvm_unreachable("bad result declaration macro kind");
-  }
+}
 
 static uint8_t getRawStableMacroIntroducedDeclNameKind(
     swift::MacroIntroducedDeclNameKind kind) {
@@ -6133,7 +6133,7 @@ void Serializer::writeAST(ModuleOrSourceFile DC) {
 
     // FIXME: Switch to a visitor interface?
     SmallVector<Decl *, 32> fileDecls;
-    nextFile->getTopLevelDecls(fileDecls);
+    nextFile->getTopLevelDeclsWithAuxiliaryDecls(fileDecls);
 
     for (auto D : fileDecls) {
       if (isa<ImportDecl>(D) || isa<IfConfigDecl>(D) ||