Turn on WITH_LLVM_CXXFILT by default

LLVM's demangler supports more modern C++ constructs such as lambdas and
unnamed types, and is actively maintained. The command line tool is
usable as a drop-in replacement for GNU c++filt, or elftoolchain's
cxxfilt. The latter is still available by using WITHOUT_LLVM_CXXFILT, if
needed.

PR: 250702
MFC after: 2 weeks