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.
Include HLSL or_intrinsic, add codegen in CGBuiltin, and the
corresponding tests in or.hlsl. Additionally, incorporate
logical-operator-errors to handle both 'and' and 'or' semantic
diagnostics.