add cxa_demangle_fuzzer

Summary:
All easy-to-find bugs in cxa_demangle where fixed now
(https://bugs.chromium.org/p/chromium/issues/detail?id=606626)
except for one (https://llvm.org/bugs/show_bug.cgi?id=31031).
Now I'd like to properly integrate this fuzzer with the source tree
and then run the fuzzer continuously on https://github.com/google/oss-fuzz

Reviewers: compnerd, mclow.lists, mehdi_amini

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28133

llvm-svn: 290650