[libcxxabi] link abort_message into unittest_demangle

unittest_demangle.pass.cpp uses the preprocessor to #include
cxa_demangle.cpp. D148566 will make more use of std::string_view in
libcxxabi rather than the home-grown StringView, but as a result of
D149092, a definition of abort_message needs to be provided.

Otherwise builds of check-cxxabi with -DLLVM_ENABLE_ASSERTIONS=ON will
fail to link with the errors:
/usr/bin/ld: /tmp/lit-tmp-0akcq37p/cc6DLdvw.o: in function `(anonymous namespace)::itanium_demangle::starts_with(std::__1::basic_string_view<char, std::__1::char_traits<char> >, char)':
unittest_demangle.pass.cpp:(.text+0x81): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-0akcq37p/cc6DLdvw.o: in function `(anonymous namespace)::itanium_demangle::starts_with(std::__1::basic_string_view<char, std::__1::char_traits<char> >, std::__1::basic_string_view<char, std::__1::char_traits<char> >)':
unittest_demangle.pass.cpp:(.text+0x2aa): undefined reference to `abort_message'
/usr/bin/ld: unittest_demangle.pass.cpp:(.text+0x312): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-0akcq37p/cc6DLdvw.o: in function `(anonymous namespace)::itanium_demangle::OutputBuffer::writeUnsigned(unsigned long, bool)':
unittest_demangle.pass.cpp:(.text+0x54f): undefined reference to `abort_message'
/usr/bin/ld: unittest_demangle.pass.cpp:(.text+0x5b7): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-0akcq37p/cc6DLdvw.o:unittest_demangle.pass.cpp:(.text+0xe6e): more undefined references to `abort_message' follow
/usr/bin/ld: /home/libcxx-builder/.buildkite-agent/builds/google-libcxx-builder-f0560ea595b1-1/llvm-project/libcxx-ci/build/generic-gcc/test/Output/unittest_demangle.pass.cpp.dir/t.tmp.exe: hidden symbol `abort_message' isn't defined

Use the preprocessor further to provide the definition of abort_message
for this unittest.

Reviewed By: #libc_abi, phosek

Differential Revision: https://reviews.llvm.org/D151160

libcxxabi/test/unittest_demangle.pass.cpp

@@ -9,6 +9,7 @@
 // UNSUPPORTED: c++03
 
 #include "../src/cxa_demangle.cpp"
+#include "../src/abort_message.cpp"
 
 using namespace __cxxabiv1;