[AST/ASTMangler] Fix linker error for SWIFT_BUILD_ONLY_SYNTAXPARSERLIB build

With such a build we avoid linking the `clangAST` library.

Author: akyrtzi

lib/AST/ASTMangler.cpp

@@ -520,6 +520,10 @@ std::string ASTMangler::mangleTypeAsUSR(Type Ty) {
 
 std::string ASTMangler::mangleDeclAsUSR(const ValueDecl *Decl,
                                         StringRef USRPrefix) {
+#if SWIFT_BUILD_ONLY_SYNTAXPARSERLIB
+  return std::string(); // not needed for the parser library.
+#endif
+
   DWARFMangling = true;
   beginManglingWithoutPrefix();
   llvm::SaveAndRestore<bool> allowUnnamedRAII(AllowNamelessEntities, true);