[clang] [llvm] [HLSL] Use hidden visibility for external linkage. (PR #140292)

[Steven Perron via cfe-commits]

s-perron wrote:

FYI: @pow2clk Here is a possible implementation for https://github.com/llvm/wg-hlsl/blob/main/proposals/0026-symbol-visibility.md. I still need to update tests, but the change seems simple enough.

https://github.com/llvm/llvm-project/pull/140292