[clang] [HLSL][Matrix] introduce MatrixElementExpr as accessor and swizzle operator (PR #171225)

[Helena Kotas via cfe-commits]

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@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -x hlsl \
+// RUN:   -ast-print -disable-llvm-passes -o - -hlsl-entry main %s \
+// RUN:   | FileCheck %s
+
+typedef float float4x4 __attribute__((matrix_type(4,4)));
+typedef float float2 __attribute__((ext_vector_type(2)));
+typedef float float3 __attribute__((ext_vector_type(3)));
+
+float4x4 gMat;
+
+[numthreads(1, 1, 1)]
+void main() {
+  float4x4 A = gMat;
+  float3 v1 = A._11_22_33;
+  float2 v2 = A._m00_m11;
----------------
hekota wrote:

```suggestion
  float3 v1 = A._12_21_32;
  float2 v2 = A._m01_m10;
```
Using different indices tests that column and row do not get accidentally mixed up.

https://github.com/llvm/llvm-project/pull/171225