[clang] [HLSL] Implement `export` keyword (PR #96823)

[Chris B via cfe-commits]

llvm-beanz wrote:

> The grammer looks exactly the same with modules. May it be compatible with modules?

The grammar is the same (although HLSL only supports the `export` keyword in a subset of the places C++ does), but the behavior isn't the same. We may look to shift HLSL's behavior to match C++ in the future but for compatibility with existing code we need to support the HLSL behavior.

https://github.com/llvm/llvm-project/pull/96823