Provide a placement new taking an ASTContext argument.

This allows more concise syntax when allocating an object using the ASTContext's allocator.
Convert a few allocations to this operator to for test purposes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62623 91177308-0d34-0410-b5e6-96231b3b80d8