[c++20] Implement most of P1152R4.

Diagnose some now-deprecated uses of volatile types:
 * as function parameter types and return types
 * as the type of a structured binding declaration
 * as the type of the lvalue operand of an increment / decrement /
   compound assignment operator

This does not implement a check for the deprecation of simple
assignments whose results are used; that check requires somewhat
more complexity and will be addressed separately.

llvm-svn: 374133