[IR] Use EXPORTAS for ARM64EC mangled symbols with dllexport attribute. (#81940)

We currently just use mangled name. This works fine, because linker
should detect that and demangle it for the export table. However, on
MSVC, the compiler is more specific and passes demangled name as well,
with EXPORTAS. This PR aims to match that. MSVC doesn't use quotes in
this case, so I added '#' to the list of characters that don't need it.