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.

llvm-svn: 224322
2 files changed