[libc++] Move compiler-detection Lit features first (#73544)

Lit features are evaluated in order. Some checks may require the
compiler detection to have run first in order to work properly, for
example some checks being added in https://reviews.llvm.org/D154246
which require GCC to have been detected. It is kind of brittle to rely
on such ordering, but in practice moving the compiler detection first
should never hurt.