[-Wunsafe-buffer-usage] Initial commit - Transition away from raw buffers.

This is the initial commit for -Wunsafe-buffer-usage, a warning that helps
codebases (especially modern C++ codebases) transition away from raw buffer
pointers.

The warning is implemented in libAnalysis as it's going to become a non-trivial
analysis, mostly the fixit part where we try to figure out if we understand
a variable's use pattern well enough to suggest a safe container/view
as a replacement. Some parts of this analsysis may eventually prove useful
for any similar fixit machine that tries to change types of variables.

The warning is disabled by default.

RFC/discussion in https://discourse.llvm.org/t/rfc-c-buffer-hardening/65734

Differential Revision: https://reviews.llvm.org/D137346