[AST] Add CanonicalDeclPtr<T>.

Summary:
CanonicalDeclPtr<T> is just like a T*, except it calls
T::getCanonicalDecl() on construction.

This is useful as the key in a "set of canonical Decls" -- it's much
less error-prone than calling getCanonicalDecl() every time you touch
the set.

Reviewers: rnk

Subscribers: cfe-commits, tra

Differential Revision: https://reviews.llvm.org/D25703

llvm-svn: 284644