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.

Work towards separating the ABI existence of libcalls vs. the
lowering selection. Set libcall selection through enums, rather
than through raw string names.

This patch avoids creating (sub x0, rhs) when lowering atomic_load_sub with a constant rhs.
Comparison with GCC: https://godbolt.org/z/c5zPdP7j4

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D158673

A target can return if a misaligned access is 'fast' as defined
by the target or not. In reality there can be different levels
of 'fast' and 'slow'. This patch changes the boolean 'Fast'
argument of the allowsMisalignedMemoryAccesses family of functions
to an unsigned representing its speed.

A target can still define it as it wants and the direct translation
of the current code uses 0 and 1 for current false and true. This
makes the change an NFC.

Subsequent patch will start using an actual value of speed in
the load/store vectorizer to compare if a vectorized access going
to be not just fast, but not slower than before.

Differential Revision: https://reviews.llvm.org/D124217

LLVM contains a helpful function for getting the size of a C-style
array: `llvm::array_lengthof`. This is useful prior to C++17, but not as
helpful for C++17 or later: `std::size` already has support for C-style
arrays.

Change call sites to use `std::size` instead.

Differential Revision: https://reviews.llvm.org/D133429

This reverts commit ef8206320769ad31422a803a0d6de6077fd231d2.

- It conflicts with the existing llvm::size in STLExtras, which will now
  never be called.
- Calling it without llvm:: breaks C++17 compat

As a conquence move llvm::array_lengthof from STLExtras.h to
STLForwardCompat.h (which is included by STLExtras.h so no build
breakage expected).

Identified with modernize-use-bool-literals.