[AST/ASTDumper] Fix linker error for SWIFT_BUILD_ONLY_SYNTAXPARSERLIB build

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

Author: akyrtzi

lib/AST/ASTDumper.cpp

@@ -3762,6 +3762,10 @@ void Type::dump() const {
 }
 
 void Type::dump(raw_ostream &os, unsigned indent) const {
+  #if SWIFT_BUILD_ONLY_SYNTAXPARSERLIB
+    return; // not needed for the parser library.
+  #endif
+
   PrintType(os, indent).visit(*this, "");
   os << "\n";
 }