Implements
https://github.com/llvm/wg-hlsl/blob/main/proposals/0026-symbol-visibility.md.
The change is to stop using the `hlsl.export` attribute. Instead,
symbols with "program linkage" in HLSL will have export linkage with
default visibility, and symbols with "external linkage" in HLSL will
have export linkage with hidden visibility.
partially fixes#70078
### Changes
- Implemented `sign` clang builtin
- Linked `sign` clang builtin with `hlsl_intrinsics.h`
- Added sema checks for `sign` to `CheckHLSLBuiltinFunctionCall` in
`SemaChecking.cpp`
- Add codegen for `sign` to `EmitHLSLBuiltinExpr` in `CGBuiltin.cpp`
- Add codegen tests to `clang/test/CodeGenHLSL/builtins/sign.hlsl`
- Add sema tests to `clang/test/SemaHLSL/BuiltIns/sign-errors.hlsl`
### Related PRs
- https://github.com/llvm/llvm-project/pull/101987
- https://github.com/llvm/llvm-project/pull/101988
### Discussion
- Should there be a `usign` intrinsic that handles the unsigned cases?
