Improve some of the conversion warnings to fire on conversion to bool.

Moves the bool bail-out down a little in SemaChecking - so now
-Wnull-conversion and -Wliteral-conversion can fire when the target type is
bool.

Also improve the wording/details in the -Wliteral-conversion warning to match
the -Wconstant-conversion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156826 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed