clang debugging code in stable/12 and current

[Thomas Eberhardt]

It looks like the enabling of debugging code in clang/llvm was flipped around during the stable/12 branch.

See the definition of NDEBUG in lib/clang/llvm.build.mk line 20:

current before r339436:
#CFLAGS+=      -DNDEBUG
after r339436:
CFLAGS+=        -DNDEBUG

stable/12 at r339435:
#CFLAGS+=      -DNDEBUG

i.e. debugging code is now disabled in current and enabled in stable/12.