Apply DXC fix and add tests

Author: joaosaffran-zz

clang/test/CodeGenHLSL/debug/rwbuffer_debug_info.hlsl

@@ -0,0 +1,8 @@
+// RUN: %clang  --driver-mode=dxc -Zi -Fc out.s -T cs_6_3 %s
+
+RWBuffer<float4> Out : register(u7, space4);
+
+[numthreads(8,1,1)]
+void main(uint GI : SV_GroupIndex) {
+  Out[GI] = 0;
+}