Basic: Add -Wswitch-bool to control warn_bool_switch_condition

This warning was missing a flag, add one.

test/Misc/warning-flags.c has one fewer entry.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203522 91177308-0d34-0410-b5e6-96231b3b80d8