[Parse] Optimize syntax parsing: Disable name lookups during parsing

This eliminates the overhead of doing name lookups during parsing (which is unncessary during syntactic parsing) by enabling the `EnableASTScopeLookup` lang option.

Author: akyrtzi

include/swift/Parse/Parser.h

@@ -637,6 +637,9 @@ class Parser {
 
   /// Add the given Decl to the current scope.
   void addToScope(ValueDecl *D) {
+    if (Context.LangOpts.EnableASTScopeLookup)
+      return;
+
     getScopeInfo().addToScope(D, *this);
   }
 

tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp

@@ -187,6 +187,8 @@ swiftparse_client_node_t SynParser::parse(const char *source) {
   LangOptions langOpts;
   langOpts.BuildSyntaxTree = true;
   langOpts.CollectParsedToken = false;
+  // Disable name lookups during parsing.
+  langOpts.EnableASTScopeLookup = true;
 
   auto parseActions =
     std::make_shared<CLibParseActions>(*this, SM, bufID);