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.
Add int overloads which cast the various ints to a float and call the
float builtin.
These overloads are conditional on hlsl version 202x or earlier.
Add tests and puts tests in own files, including some of the tests added
for double overloads.
Closes#128229