[cmake] Disable GCC 9's -Wpessimizing-move

Similar to the existing code for disabling GCC's -Wredudant-move,
also check for the -Wpessimizing-move option and disable it if
possible.

This silences another bunch of noisy warnings when building LLVM
with GCC 9.

As noted for -Wredundant-move, the code can't be fixed to silence the
warnings while retaining support for older compilers.

Differential Revision: https://reviews.llvm.org/D98942