[OpenMP] Support nested OpenMP context selectors (declare variant)

Due to `omp begin/end declare variant`, OpenMP context selectors can be
nested. This patch adds initial support for this so we can use it for
target math variants. We should improve the detection of "equivalent"
scores and user conditions, we should also revisit the data structures
of the OMPTraitInfo object, however, both are not pressing issues right
now.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D85877