[NFC] RuntimeLibcalls: Prefix the impls with 'Impl_' (#153850)

As noted in #153256, TableGen is generating reserved names for
RuntimeLibcalls, which resulted in a build failure for Arm64EC since
`vcruntime.h` defines `__security_check_cookie` as a macro.

To avoid using reserved names, all impl names will now be prefixed with
`Impl_`.

`NumLibcallImpls` was lifted out as a `constexpr size_t` instead of
being an enum field.

While I was churning the dependent code, I also removed the TODO to move
the impl enum into its own namespace and use an `enum class`: I
experimented with using an `enum class` and adding a namespace, but we
decided it was too verbose so it was dropped.

0 files changed, 0 insertions, 0 deletions