Check for bool-like conversion in conditional expressions.

Add a check for bool-like conversions for the condition expression of
conditional operators.  This is similiar to the checking of condition
expressions of if statements, for-loops, while-loops, and do-while loops.
Specificially, this is to fix the problem of assert("message") not triggering
-Wstring-conversion when the assert macro uses a conditional operator.

llvm-svn: 237856