Sema: Don't crash converting to bool from _Atomic

Turning our _Atomic L-value into an R-value removes its _Atomic-ness.
However, we didn't update our 'FromType' which made
ScalarTypeToBooleanCastKind think we were trying to pass it a
non-scalar.

This fixes PR21836.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224322 91177308-0d34-0410-b5e6-96231b3b80d8