[FunctionAttrs] Make the per-function attribute inference a boring

static function rather than a method. It just needed access to
TargetLibraryInfo, and this way it can be easily reused between the
current FunctionAttrs implementation and any port for the new pass
manager.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247522 91177308-0d34-0410-b5e6-96231b3b80d8