Add LLVMContext::emitWarning methods and use them. <rdar://problem/12867368>

When the backend is used from clang, it should produce proper diagnostics
instead of just printing messages to errs(). Other clients may also want to
register their own error handlers with the LLVMContext, and the same handler
should work for warnings in the same way as the existing emitError methods.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171041 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -1154,8 +1154,9 @@ bool InstCombiner::transformConstExprCastCall(CallSite CS) {
   // If we are removing arguments to the function, emit an obnoxious warning.
   if (FT->getNumParams() < NumActualArgs) {
     if (!FT->isVarArg()) {
-      errs() << "WARNING: While resolving call to function '"
-             << Callee->getName() << "' arguments were dropped!\n";
+      FT->getContext().emitWarning("while resolving call to function '" +
+                                   Callee->getName() +
+                                   "' arguments were dropped");
     } else {
       // Add all of the arguments in their promoted form to the arg list.
       for (unsigned i = FT->getNumParams(); i != NumActualArgs; ++i, ++AI) {