Generalize DWARF tests that expect DW_FORM_strp

The modified tests were failing on AIX because DWARF on AIX uses inline strings by default, but the tests check for `DW_FORM_strp`.

This patch removes `DW_FORM_strp` so both forms will pass the check.

Reviewed By: shchenz, dblaikie

Differential Revision: https://reviews.llvm.org/D112286