[demangler] Add co_await demangling

The demangler doesn't understand 'aw' as an operator name. This adds
the necessary smarts -- you may use this as an operator functionname,
but not as an expression operator.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D120143

GitOrigin-RevId: 75db1795e44629b9c84c70410f6a6b106c4c5f3a
2 files changed