Volatile reads are side-effecting operations, but in the general case of...

Volatile reads are side-effecting operations, but in the general case of access through a volatile-qualified type, we're not certain of the underlying object's side-effects on access.

Treat volatile accesses as "maybe" instead of "definite" side effects for the purposes of warning on evaluations in an unevaluated context. No longer diagnose on idiomatic code like:

int * volatile v;
(void)sizeof(*v);

llvm-svn: 225116