llvm-cxxfilt: Demangle gcc "old-style unified" ctors and dtors

These are variant 4, cf
https://github.com/gcc-mirror/gcc/blob/master/gcc/cp/mangle.c#L1851
https://github.com/gcc-mirror/gcc/blob/master/gcc/cp/mangle.c#L1880
and gcc seems to sometimes emit them still.

Differential Revision: https://reviews.llvm.org/D60229

llvm-svn: 357645