[libc++abi] Avoid including source files into unittest_demangle

That is not necessary to test what we're testing, and in fact including
abort_message.cpp into that file caused some link errors if we didn't
link some of the dependencies of libc++abi directly into the test.

Differential Revision: https://reviews.llvm.org/D152862

GitOrigin-RevId: a86df48c38e8138ad67050967cef63548bffb230
1 file changed