[XRay] XRAY_NEVER_INSTRUMENT more functions, consolidate allocators

Summary:
In this change we apply `XRAY_NEVER_INSTRUMENT` to more functions in the
profiling implementation to ensure that these never get instrumented if
the compiler used to build the library is capable of doing XRay
instrumentation.

We also consolidate all the allocators into a single header
(xray_allocator.h) which sidestep the use of the internal allocator
implementation in sanitizer_common.

This addresses more cases mentioned in llvm.org/PR38577.

Reviewers: mboerger, eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51776

llvm-svn: 341647