Allow paren casted throw statements inside of ternary expressions

clang would incorrectly not allow the following:

int x = true ? (throw 1) : 2;

The problem exists because we don't see beyond the parens.
This, in turn, causes us to believe that we are choosing between void
and int which we diagnose as an error.

Instead, allow clang to see the 'throw' inside the parens.

llvm-svn: 183085