The Unsupported case is special and doesn't have an entry in the
vector, and is directly emitted as the 0 case. This should be
harmless as it is, but could break if the right number of new
libcalls is added.

This is a step towards separating the set of available libcalls
from the lowering decision of which call to use. Libcall recognition
now directly checks availability instead of indirectly checking through
the lowering table.

This reverts 9c361cc and replaces f490dbdc. Instead of using the lambda
to try avoid naming the variables, just disambiguate the different
AlwaysAvailable
sets with the calling convention name.

This change resolves a stack usage issue seen in the TableGen'd function
`setTargetRuntimeLibcallSets` when compiled with MSVC. This change
reduces the frame size from 47720 bytes to 48 bytes.

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.

We were sizing the table appropriately for the number of LibcallImpls,
but many of those have identical names which were pushing up the
collision count unnecessarily. This ends up decreasing the table size
slightly, and makes it a bit faster.

BM_LookupRuntimeLibcallByNameRandomCalls improves by ~25% and
BM_LookupRuntimeLibcallByNameSampleData by ~5%.

As a secondary change, align the table size up to the next
power of 2. This makes the table larger than before, but improves
the sample data benchmark by an additional 5%.

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 reverts commit 334e9bf2dd01fbbfe785624c0de477b725cde6f2.

Check if llvm-nm exists before building the benchmark.

…210)"

This reverts commit 9a14b1d254a43dc0d4445c3ffa3d393bca007ba3.

Revert "RuntimeLibcalls: Return StringRef for libcall names (#153209)"

This reverts commit cb1228fbd535b8f9fe78505a15292b0ba23b17de.

Revert "TableGen: Emit statically generated hash table for runtime
libcalls (#150192)"

This reverts commit 769a9058c8d04fc920994f6a5bbb03c8a4fbcd05.

Reverted three changes because of a CMake error while building llvm-nm
as reported in the following PR:
https://github.com/llvm/llvm-project/pull/150192#issuecomment-3192223073