[Clang][DirectX] Always use Diagnostic Printer

fixes #135654

In #128613 we added safe guards to prevent the lowering of just any
intrinsic in the backend. We used `DiagnosticInfoUnsupported` to do
this.

What we found was when using `opt` the diagnostic printer was called
but when using clang the diagnostic message was called.

Printing message in the clang version means we miss valuable debugging
information like function name and function type when LLVMContext was
only needed to call `getBestLocationFromDebugLoc`.

Author: farzonl

clang/lib/CodeGen/CodeGenAction.cpp

@@ -605,13 +605,11 @@ void BackendConsumer::UnsupportedDiagHandler(
 
   // Context will be nullptr for IR input files, we will construct the diag
   // message from llvm::DiagnosticInfoUnsupported.
-  if (Context != nullptr) {
+  if (Context != nullptr)
     Loc = getBestLocationFromDebugLoc(D, BadDebugInfo, Filename, Line, Column);
-    MsgStream << D.getMessage();
-  } else {
-    DiagnosticPrinterRawOStream DP(MsgStream);
-    D.print(DP);
-  }
+
+  DiagnosticPrinterRawOStream DP(MsgStream);
+  D.print(DP);
 
   auto DiagType = D.getSeverity() == llvm::DS_Error
                       ? diag::err_fe_backend_unsupported

clang/test/CodeGenDirectX/unsupported_reduc_mul.hlsl

@@ -0,0 +1,7 @@
+// RUN: not %clang_dxc -T lib_6_3 %s 2>&1 | FileCheck %s
+
+// CHECK: error: <unknown>:0:0: in function llvm.vector.reduce.mul.v4i32 i32 (<4 x i32>): Unsupported intrinsic for DXIL lowering
+
+export int vecReduceMulTest(int4 vec) {
+    return __builtin_reduce_mul(vec);
+}