[OpenMP] `omp begin/end declare variant` - part 1, parsing

This is the first part extracted from D71179 and cleaned up.

This patch provides parsing support for `omp begin/end declare variant`,
as defined in OpenMP technical report 8 (TR8) [0].

A major purpose of this patch is to provide proper math.h/cmath support
for OpenMP target offloading. See PR42061, PR42798, PR42799. The current
code was developed with this feature in mind, see [1].

[0] https://www.openmp.org/wp-content/uploads/openmp-TR8.pdf
[1] https://reviews.llvm.org/D61399#change-496lQkg0mhRN

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D74941