MFC r315947:

Import libcxxrt master 8a853717e61d5d55cbdf74d9d0a7545da5d5ff92.

Interesting fixes which were not already merged:
0c7c611 Merge C++ demangler bug fixes from ELF Tool Chain (#40)
2b208d9 __cxa_demangle_gnu3: demangle 'z' as '...', not 'ellipsis' (#41)

MFC r303400: libcxxrt: fix demangling of wchar_t

'wchar_t' is 7 characters long, not 6. r303297 fixed this (in head) in
libelftc, but not the second copy of this file that we have in libcxxrt.

PR: 208661

MFC r284551: Import libcxxrt master e64e93f.

This includes a number of demangler fixes obtained from upstream
ELF Tool Chain.

PR: 200913
Sponsored by: The FreeBSD Foundation

MFC r284551: Import libcxxrt master e64e93f.

This includes a number of demangler fixes obtained from upstream
ELF Tool Chain.

PR: 200913
Sponsored by: The FreeBSD Foundation