Port libcxxabi r344607 into llvm

Summary:
The original commit message was:
    This uses CRTP (for performance reasons) to allow a user the override
    demangler functions to implement custom parsing logic. The motivation
    for this is LLDB, which needs to occasionaly modify the mangled names.
    One such instance is already implemented via the TypeCallback member,
    but this is very specific functionality which does not help with any
    other use case. Currently we have a use case for modifying the
    constructor flavours, which would require adding another callback. This
    approach does not scale.

    With CRTP, the user (LLDB) can override any function it needs without
    any special support from the demangler library. After LLDB is ported to
    use this instead of the TypeCallback mechanism, the callback can be
    removed.

The only difference here is the addition of a unit test which exercises
the CRTP mechanism to override a function in the parser.

Reviewers: erik.pilkington, rsmith, EricWF

Subscribers: mgorny, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D53300

llvm-svn: 344703