[clang] [HLSL] add loop unroll (PR #93879)

[Chris B via cfe-commits]

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llvm-beanz wrote:

Can you also add some tests for nested loops?

Cases like:
```hlsl
[unroll]
for( int i = 0; i < 100; ++i) { // unroll this
  for( int j = 0; j < 100; ++j) { // but not this
  }
}
```

https://github.com/llvm/llvm-project/pull/93879