[OpenMP] Encode `omp [...] assume[...]` assumptions with `omp[x]` prefix

[OpenMP] Encode `omp [...] assume[...]` assumptions with `omp[x]` prefix

Since these assumptions are coming from OpenMP it makes sense to mark
them as such in the generic IR encoding. Standardized assumptions will
be named
  omp_ASSUMPTION_NAME
and extensions will be named
  ompx_ASSUMPTION_NAME
which is the OpenMP 5.2 syntax for "extensions" of any kind.

This also matches what the OpenMP-Opt pass expects.

Summarized,
  #pragma omp [...] assume[s] no_parallelism
now generates the same IR assumption annotation as
  __attribute__((assume("omp_no_parallelism")))

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D105937