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.

Also starts pruning out these calls if the exception model is
forced to none.

I worked backwards from the logic in addPassesToHandleExceptions
and the pass content. There appears to be some tolerance
for mixing and matching exception modes inside of a single module.
As far as I can tell _Unwind_CallPersonality is only relevant for
wasm, so just add it there.

As usual, the arm64ec case makes things difficult and is
missing test coverage. The set of calls in list form is necessary
to use foreach for the duplication, but in every other context a
dag is more convenient. You cannot use foreach over a dag, and I
haven't found a way to flatten a dag into a list.

This removes the last manual setLibcallImpl call in generic code.

This introduced a 5% compile-time regression on AArch64, see
https://llvm-compile-time-tracker.com/compare.php?from=b9138bde3562de5c28a239dbd303caf2406678c6&to=271688b87abe7cf45aceaff8266270a25eb7b436&stat=instructions:u.

Reverts llvm/llvm-project#152505.

This updates everywhere we emit/check an SME routines to use
RuntimeLibcalls to get the function name and calling convention.

Note: RuntimeLibcallEmitter had some issues with emitting non-unique
variable names for sets of libcalls, so I tweaked the output to avoid
the need for variables.

We still need to manually set the calling conventions of
some libcalls until the lowering is separated out.

Allow associating a non-default CallingConv with a set of library
functions, and applying a default for a SystemLibrary.

I also wanted to be able to apply a default calling conv
to a RuntimeLibcallImpl, but that turned out to be annoying
so leave it for later.