Don't build expressions for invalid casts.

This matches how we normally perform semantic analysis for other sorts
of invalid expressions: it means we don't have to reason about invalid
sub-expressions.

Fixes PR16680.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187276 91177308-0d34-0410-b5e6-96231b3b80d8