8ea07101 "external/firebird: Work around
operator new alignment violations" had misused DEBUG_GDS_ALLOC to work around
the problem that recent Clang on Linux x86-64 assumes some storage allocated via
Firebird's global operator new replacement to be 16-byte aligned, while Firebird
only provides 8-byte alignment.

This problem now also appears on macOS x86-64, at least with Apple's Clang
version "Apple LLVM version 9.1.0 (clang-902.0.39.1)" from Xcode 9.3 (as well as
with recent plain Clang trunk, towards Clang 7), when building --enable-
optimized.  However, while the DEBUG_GDS_ALLOC hack would still cause
ICU_convert_init (workdir/UnpackedTarball/firebird/temp/Release/intl/cv_icu.o)
to not use movaps on erroneously assumed to be 16-byte aligned memory, it would
cause strange failures on macOS (pos being out of bounds in traRpbList::PopRpb,
workdir/UnpackedTarball/firebird/src/jrd/rpb_chain.cpp, in the invocation of
isql during the build of external/firebird) that I haven't tracked down further.

As it happens, the recently added 14955ed9
"external/firebird: Support Clang ASan" provides a different workaround for the
underlying problem that appears to work well on both Linux and macOS x86-64,
reusing USE_ASAN also in these cases to shut down most of Firebird's own memory
management.

I assume that affected Clang are plain Clang >= 4 (as I'd mentioned in my
<https://sourceforge.net/p/firebird/mailman/message/35671804/> "Re: [Firebird-
devel] alloc.h global operator new replacement violating alignment
requirements") and Apple Clang >= 9 (for which __apple_build_version__ is
defined).

Because DEBUG_GDS_ALLOC is no longer passed in from the outside, its setting in
external/firebird/asan.patch can be simplified (cf. commit message of
14955ed9).

(The given scenario in ICU_convert_init involves an allocation of 24 bytes,
where Clang may or may not be allowed to assume 16-byte alignment, see
<http://lists.llvm.org/pipermail/cfe-dev/2018-April/057669.html> "Re: [cfe-dev]
operator new alignment assumptions".  However, as reported at
<https://sourceforge.net/p/firebird/mailman/message/35671750/> "Re: [Firebird-
devel] alloc.h global operator new replacement violating alignment
requirements", Firebird only supports 8-byte alignment, which would definitely
be wrong in a similar scenario where the requested allocation was a multiple of
16 bytes.)

Change-Id: I48884f9d008eaeaea369850e24f05b8806f9b676
Reviewed-on: https://gerrit.libreoffice.org/52956Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>