The mem* builtins are often used (or should be used) in places where time-of-check time-of-use security issues are important (e.g. copying from untrusted buffers), because it prevents multiple reads / multiple writes from occurring at the untrusted memory location. The current builtins don't accept volatile pointee parameters in C++, and merely warn about such parameters in C, which leads to confusion. In these settings, it's useful to overload the builtin and permit volatile pointee parameters. The code generation then directly emits the existing volatile variant of the mem* builtin function call, which ensures that the affected memory location is only accessed once (thereby preventing double-reads under an adversarial memory mapping).
Instead of just handling volatile, this patch handles other overloads when they make sense, specifically around pointer qualification. It also cleans up some of the custom diagnostic handling to reduce duplication.
The patch also happens to be a decent base to implement C's memset_s from Annex K, as well as an alternate (and arguably better approach) that the C++ Committee's proposal for byte-wise atomic memcpy as described in https://wg21.link/P1478 (in particular, it lets developer own the fences, a topic discussed in https://wg21.link/p2061).
Side-note: yes, ToCToU avoidance is a valid use for volatile https://wg21.link/p1152r0#uses.
RFC for this patch: http://lists.llvm.org/pipermail/cfe-dev/2020-May/065385.html