Ensure AtomicExpr goes through SEMA checking after TreeTransform

RebuildAtomicExpr was skipping doing semantic analysis which broke in
the cases where the expressions were not dependent. This resulted in the
ImplicitCastExpr from an array to a pointer being lost, causing a crash
in IR CodeGen.

Differential Revision: https://reviews.llvm.org/D67854

llvm-svn: 372422