[CSDiagnostics] Fix requirement source lookup to support member references

Fuzzing found a problem related to re-typecheck introducing
`MemberRefExpr` into AST with failing requirements which
are then diagnosed via fixes, quite unlikely case to be
found in the wild.

(cherry picked from commit 03d2831)

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -119,6 +119,10 @@ ValueDecl *RequirementFailure::getDeclRef() const {
     ConstraintLocatorBuilder subscript(locator);
     locator = cs.getConstraintLocator(
         subscript.withPathElement(PathEltKind::SubscriptMember));
+  } else if (isa<MemberRefExpr>(anchor)) {
+    ConstraintLocatorBuilder memberRef(locator);
+    locator =
+        cs.getConstraintLocator(memberRef.withPathElement(PathEltKind::Member));
   }
 
   auto overload = getOverloadChoiceIfAvailable(locator);