[HLSL] enable Wconversion by default for HLSL (llvm#166617)

Add WConversion by default to cc1 args.  
Closes llvm#134761

Author: spall

clang/lib/Driver/ToolChains/HLSL.cpp

@@ -567,3 +567,7 @@ bool HLSLToolChain::isLastJob(DerivedArgList &Args,
   // output to the result file.
   return true;
 }
+
+void HLSLToolChain::addClangWarningOptions(ArgStringList &CC1Args) const {
+  CC1Args.push_back("-Wconversion");
+}

clang/lib/Driver/ToolChains/HLSL.h

@@ -91,6 +91,8 @@ class LLVM_LIBRARY_VISIBILITY HLSLToolChain : public ToolChain {
   // Set default DWARF version to 4 for DXIL uses version 4.
   unsigned GetDefaultDwarfVersion() const override { return 4; }
 
+  void addClangWarningOptions(llvm::opt::ArgStringList &CC1Args) const override;
+
 private:
   mutable std::unique_ptr<tools::hlsl::Validator> Validator;
   mutable std::unique_ptr<tools::hlsl::MetalConverter> MetalConverter;

clang/test/Driver/HLSL/wconversion.hlsl

@@ -0,0 +1,7 @@
+// RUN: %clang_dxc -T lib_6_7 %s -### %s 2>&1 | FileCheck %s --check-prefixes=CONV
+// RUN: %clang_dxc -T lib_6_7 -Wno-conversion %s -### %s 2>&1 | FileCheck %s --check-prefixes=NOCONV
+
+// make sure we generate -Wconversion by default
+// CONV: "-Wconversion"
+// make sure -Wno-conversion still works
+// NOCONV: "-Wno-conversion"