[AST] Remove respectOriginallyDefinedIn parameter from mangleAnyDecl

This was always set to `true` except for USR mangling, where we already
have it set to `false` for IDE USRs. The other clients were:

- AutoDiff, which is just using the resulting string as a dictionary
key, so don't seem to have any preference.
- The ClangImporter, which always overrides `@_originallyDefinedIn`
anyway.

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -155,8 +155,7 @@ static std::string getMangledNameString(const Decl *D) {
   if (!VD)
     return std::string();
   Mangle::ASTMangler mangler(VD->getASTContext());
-  return mangler.mangleAnyDecl(VD, /*prefix=*/true,
-                               /*respectOriginallyDefinedIn=*/true);
+  return mangler.mangleAnyDecl(VD, /*addPrefix*/ true);
 }
 
 static std::string getTypeString(const ValueDecl *VD) {