[IDE] Enable 'IgnoreSwiftSourceInfo' in code completion

Get source locations from '.swiftsourceinfo' is currently expensive.
Since they are not necessary for code completion, set
'IgnoreSwiftSourceInfo' to 'true'.

rdar://problem/57194358
https://bugs.swift.org/browse/SR-11767

tools/SourceKit/lib/SwiftLang/SwiftConformingMethodList.cpp

@@ -67,6 +67,10 @@ static bool swiftConformingMethodListImpl(
     return false;
   }
 
+  // Disable source location resolutions from .swiftsourceinfo file because
+  // they are somewhat heavy operations and are not needed for completions.
+  Invocation.getFrontendOptions().IgnoreSwiftSourceInfo = true;
+
   Invocation.setCodeCompletionPoint(newBuffer.get(), Offset);
 
   // Create a factory for code completion callbacks that will feed the