[analyzer] Make inlining decisions based on the callee being variadic.

...rather than trying to figure it out from the call site, and having
people complain that we guessed wrong and that a prototype-less call is
the same as a variadic call on their system. More importantly, fix a
crash when there's no decl at the call site (though we could have just
returned a default value).

<rdar://problem/15037033>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191599 91177308-0d34-0410-b5e6-96231b3b80d8