Revert "Use InernalAlloc in DemangleCXXABI"

This reverts commit 649004ae.

This commit caused regression in TSAN tests on Darwin:
https://green.lab.llvm.org/green/job/clang-stage1-RA/35380/

These tests have been failing:
SanitizerCommon-tsan-x86_64-Darwin.Darwin.symbolizer-function-offset-dladdr.cpp
SanitizerCommon-tsan-x86_64h-Darwin.Darwin.symbolizer-function-offset-dladdr.cpp
ThreadSanitizer-x86_64.Darwin.symbolizer-dladdr.cpp
ThreadSanitizer-x86_64h.Darwin.symbolizer-dladdr.cpp