[ConstraintSystem] Adjust overload filtering to be less aggressive in code completion mode

Avoid filtering overloads for calls which have code completion
token as an argument, that would always undesired results,
because such calls could be missing arguments before or after
the token.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -8531,6 +8531,12 @@ bool ConstraintSystem::simplifyAppliedOverloadsImpl(
     Constraint *disjunction, TypeVariableType *fnTypeVar,
     const FunctionType *argFnType, unsigned numOptionalUnwraps,
     ConstraintLocatorBuilder locator) {
+  // Don't attempt to filter overloads when solving for code completion
+  // because presence of code completion token means that any call
+  // could be malformed e.g. missing arguments e.g. `foo([.#^MEMBER^#`
+  if (isForCodeCompletion())
+    return false;
+
   if (shouldAttemptFixes()) {
     auto arguments = argFnType->getParams();
     bool allHoles =