[libcxxabi] Add test to assert that ItaniumDemangle.h is the same (#140323)

ItaniumDemangle.h exists in both llvm/ and libcxxabi/. These files are
supposed to be copies of each other (minus the top two lines). This
patch adds a test to assert that this is the case to enable tooling to
automatically detect this as an issue, like in #139825. This makes it
easier for contributors unfamiliar with the duplication to make
changes/get appropriate reviews.

Ideally we would share the file and copy it from one place to the other
but the ideal way to do this (based on previous discussion with libc++
maintainers) would be a new runtime library that clearly outlines
requirements, so that is left for later with the test being used as a
stopgap. This is a relatively common approach for structures shared
between compiler-rt and LLVM.

This patch does make the test reference the LLVM source directory, but
that should be fine given building libcxxabi is only supported through
the runtimes build in the monorepo meaning it should always be
available.

Author: boomanaiden154

libcxxabi/test/configs/cmake-bridge.cfg.in

@@ -27,6 +27,8 @@ config.test_exec_root = os.path.join('@LIBCXXABI_BINARY_DIR@', 'test')
 config.host_triple = '@LLVM_HOST_TRIPLE@'
 
 config.substitutions.append(('%{libcxx}', '@LIBCXXABI_LIBCXX_PATH@'))
+config.substitutions.append(('%{libcxxabi}', '@LIBCXXABI_SOURCE_DIR@'))
+config.substitutions.append(('%{llvm}', '@LLVM_MAIN_SRC_DIR@'))
 config.substitutions.append(('%{install-prefix}', '@LIBCXXABI_TESTING_INSTALL_PREFIX@'))
 config.substitutions.append(('%{include}', '@LIBCXXABI_TESTING_INSTALL_PREFIX@/include'))
 config.substitutions.append(('%{cxx-include}', '@LIBCXXABI_TESTING_INSTALL_PREFIX@/@LIBCXXABI_INSTALL_INCLUDE_DIR@'))

libcxxabi/test/itanium_demangle_matches_llvm.sh.test

@@ -0,0 +1,6 @@
+# This test diffs the ItaniumDemangle.h header in libcxxabi and LLVM to ensure
+# that they are the same.
+
+# RUN: tail -n +3 %{libcxxabi}/src/demangle/ItaniumDemangle.h > %t.libcxxabi_demangle
+# RUN: tail -n +3 %{llvm}/include/llvm/Demangle/ItaniumDemangle.h > %t.llvm_demangle
+# RUN: diff %t.libcxxabi_demangle %t.llvm_demangle