[OpenMP] Split OpenMP/target_map_codegen test [NFC]

The test file is the single longest test among clang's tests and ends up about
doubling the wall time of clang tests on machines with high number of cores.

The test appears to consist of multiple independent subtests and does not have
to be in one file. Splitting it into smaller parts reduces test time on my
machine from ~80s down to ~45.

Differential Revision: https://reviews.llvm.org/D85551