Sync libiberty with gcc and add Rust demangling support to cxxfilt.

[Mark Wielaard]

The following patch series syncs libiberty (and include/demangle.h)
with gcc and enables the new Rust symbol demangling support in c++filt.

OK to push these patches?

 [PATCH 01/10] PR c++/71696 testcase.
 [PATCH 02/10] libiberty: Fix memory leak in ada_demangle when symbol
 [PATCH 03/10] Implement P0136R1, Rewording inheriting constructors.
 [PATCH 04/10] libiberty: Fix -Wimplicit-fallthrough warnings.
 [PATCH 05/10] Implement P0012R1, Make exception specifications part
 [PATCH 06/10] libiberty: Add -Wshadow=local to warning flags (if
 [PATCH 07/10] libiberty: Fix some demangler crashes caused by reading
 [PATCH 08/10] libiberty: demangler crash with missing :? or fold
 [PATCH 09/10] libiberty: Add Rust symbol demangling.
 [PATCH 10/10] cxxfilt: Recognize rust_demangling.