[AMDGPU][OpenMP] Emit textual IR for -emit-llvm -S

Previously clang would print a binary blob into the bundled file
for amdgcn. With this patch, it will instead print textual IR as
expected.

Reviewed By: JonChesterfield, ronlieb

Differential Revision: https://reviews.llvm.org/D102065

Change-Id: I10c0127ab7357787769fdf9a2edd4b3071e790a1