cxa_demangle: make demangler's parsing functions overridable

Summary:
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.

More context can be found in D50599.

Reviewers: erik.pilkington, rsmith

Subscribers: christof, ldionne, llvm-commits, libcxx-commits

Differential Revision: https://reviews.llvm.org/D52992

llvm-svn: 344607

libcxxabi/src/cxa_demangle.cpp

@@ -324,7 +324,7 @@ public:
 // Code beyond this point should not be synchronized with LLVM.
 //===----------------------------------------------------------------------===//
 
-using Demangler = itanium_demangle::Db<DefaultAllocator>;
+using Demangler = itanium_demangle::ManglingParser<DefaultAllocator>;
 
 namespace {
 enum : int {