Add a warning for implicit conversion from function literals (and static

Add a warning for implicit conversion from function literals (and static
methods) to bool. E.g.

void foo() {}
if (f) { ... // <- Warns here.
}

Only applies to non-weak functions, and does not apply if the function address
is taken explicitly with the addr-of operator.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145849 91177308-0d34-0410-b5e6-96231b3b80d8