[HLSL] Allow non .hlsl files as source files (llvm#137378)

Changes the driver to assume input file with an unknown extension are
HLSL source files instead of object files.

Fixes llvm#137370

Author: s-perron

clang/lib/Driver/Driver.cpp

@@ -3001,6 +3001,8 @@ void Driver::BuildInputs(const ToolChain &TC, DerivedArgList &Args,
               Ty = types::TY_CXX;
             else if (CCCIsCPP() || CCGenDiagnostics)
               Ty = types::TY_C;
+            else if (IsDXCMode())
+              Ty = types::TY_HLSL;
             else
               Ty = types::TY_Object;
           }

clang/test/Driver/dxc_I.test

@@ -0,0 +1,4 @@
+// RUN: %clang_dxc -Tlib_6_3  -### %s 2>&1 | FileCheck %s
+
+// Make sure a non `.hlsl` file is considered an HLSL source file in dxc mode.
+// CHECK: "-x" "hlsl" "{{.*}}dxc_I.test"