NFC: Make the copies of the demangler byte-for-byte identical

With this patch, the copies of the files ItaniumDemangle.h,
StringView.h, and Utility.h are kept byte-for-byte in sync between
libcxxabi and llvm. All differences (namespaces, fallthrough, and
unreachable macros) are defined in each copies' DemanglerConfig.h.

This patch also adds a script to copy changes from libcxxabi
(cp-to-llvm.sh), and a README.txt explaining the situation.

Differential revision: https://reviews.llvm.org/D53538

llvm-svn: 351474