[clang] [clang] Add `::_placement_new` expression for built-in global placement new (PR #72209)

[David Blaikie via cfe-commits]

dwblaikie wrote:

Might be worth cconsidering the points in "Contributing Extensions to Clang" here: https://clang.llvm.org/get_involved.html

It'd be great if there were some numbers/metrics associated with the benefit, ideally on some open source/commonly accessible codebase.

https://github.com/llvm/llvm-project/pull/72209