Restore global descriptor demangling after D138095 "[asan] Keep Itanium mangled names...

Restore global descriptor demangling after D138095 "[asan] Keep Itanium mangled names in global metadata"

This amends commit 00be3578e0841dd9abe408e5b4946180de0bf46b to demangle symbol
names in global descriptors. We keep the mangled name for the `__odr_gen_asan_*`
variables and the runtime __cxa_demangle call site change (which fixed possible
leaks for other scenarios: non-fatal diagnostics).

compiler-rt/lib/sanitizer_common/sanitizer_symbolizer_posix_libcdep.cpp uses
an undefined weak `__cxa_demangle` which does not pull in an archive definition.
A -static-libstdc++ executable link does not get demangled names.

Unfortunately this means we cannot rely on runtime demangling.
See compiler-rt/test/asan/TestCases/global-demangle.cpp