[Index] Fast path for getting value member

Avoid solving constraint system if there's only one viable overload.
This should also avoids a crash we are seeing in
'candidates.size() == 1' branch in the constraint system.

rdar://problem/64636688

Author: rintaro

lib/Sema/CSGen.cpp

@@ -4409,6 +4409,13 @@ swift::resolveValueMember(DeclContext &DC, Type BaseTy, DeclName Name) {
   if (LookupResult.ViableCandidates.empty())
     return Result;
 
+  // If there's only one viable member, that is the best one.
+  if (LookupResult.ViableCandidates.size() == 1) {
+    Result.Impl->BestIdx = Result.Impl->AllDecls.size();
+    Result.Impl->AllDecls.push_back(LookupResult.ViableCandidates[0].getDecl());
+    return Result;
+  }
+
   // Try to figure out the best overload.
   ConstraintLocator *Locator = CS.getConstraintLocator({});
   TypeVariableType *TV = CS.createTypeVariable(Locator,