Compute standard conversion sequences for conversions to atomic

types. The second and third conversions in the sequence are based on
the conversion for the underlying type, so that we get sensible
overloading behavior for, e.g., _Atomic(int) vs. _Atomic(float).

As part of this, actually implement the lvalue-to-rvalue conversion
for atomic types. There is probably a pile of code in SemaExpr that
can now be deleted, but I haven't tracked it down yet.

llvm-svn: 154596