Add regression test for debugging __fp16, float, and double constants

See https://android-review.googlesource.com/#/c/312566/

NOTE: Upstream llvm/clang has changes "DebugInfo: New metadata
representation for global variables" (D20147, D20415) that rework
enough of the debug emission that this test case no longer makes sense
(in particular, the bitcode that forms part of this test case no
longer represents what slang will emit).  Instead there is a separate
patch to make a corresponding fix (D27549, D27589).

We should throw away this patch when we incorporate those upstream
changes.

Test: full_fugu-eng: libbcc/tests

Change-Id: I56325195f447dccf5d9866380a5766476bed5e97