[X32] Add Triple::isX32(), use it.

So far, support for x86_64-linux-gnux32 has been handled by explicit
comparisons of Triple.getEnvironment() to GNUX32. This worked as long as
x86_64-linux-gnux32 was the only X32 environment to worry about, but we
now have x86_64-linux-muslx32 as well. To support this, this change adds
an isX32() function and uses it. It replaces all checks for GNUX32 or
MuslX32 by isX32(), except for the following:

- Triple::isGNUEnvironment() and Triple::isMusl() are supposed to treat
  GNUX32 and MuslX32 differently.
- computeTargetTriple() needs to be able to transform triples to add or
  remove X32 from the environment and needs to map GNU to GNUX32, and
  Musl to MuslX32.
- getMultiarchTriple() completely lacks any Musl support and retains the
  explicit check for GNUX32 as it can only return x86_64-linux-gnux32.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D103777