[Driver][DXC] Treat stdin as HLSL

Differential Revision: https://reviews.llvm.org/D157562

Author: bogner

clang/lib/Driver/Driver.cpp

@@ -2648,6 +2648,8 @@ void Driver::BuildInputs(const ToolChain &TC, DerivedArgList &Args,
         if (memcmp(Value, "-", 2) == 0) {
           if (IsFlangMode()) {
             Ty = types::TY_Fortran;
+          } else if (IsDXCMode()) {
+            Ty = types::TY_HLSL;
           } else {
             // If running with -E, treat as a C input (this changes the
             // builtin macros, for example). This may be overridden by -ObjC

clang/test/Driver/dxc_stdin.hlsl

@@ -0,0 +1,3 @@
+// RUN: %clang_dxc -Tlib_6_7 - -### 2>&1 | FileCheck %s
+// CHECK: "-cc1"
+// CHECK-SAME: "-x" "hlsl" "-"