[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

@@ -1491,6 +1491,10 @@ void ASTMangler::appendImplFunctionType(SILFunctionType *fn) {
 Optional<ASTMangler::SpecialContext>
 ASTMangler::getSpecialManglingContext(const ValueDecl *decl,
                                       bool useObjCProtocolNames) {
+  #if SWIFT_BUILD_ONLY_SYNTAXPARSERLIB
+    return None; // not needed for the parser library.
+  #endif
+
   // Declarations provided by a C module have a special context mangling.
   //   known-context ::= 'So'
   //