AddGlobalAnnotations for function with or without function body.

When AnnotateAttr is on a function, AddGlobalAnnotations is only called
in CodeGenModule::EmitGlobalFunctionDefinition which means AnnotateAttr
on function declaration without function body will be ignored.
The patch will move AddGlobalAnnotations  to
CodeGenModule::SetFunctionAttributes, so with or without function body,
the AnnotateAttr will get code gen for a function.

It'll help case when AnnotateAttr is on external function, and the
AnnotateAttr will be consumed in IR level.

For example, a pass to collect num of uses for functions with
__attribute((annotate("count_use"))) after optimizations,
As long as there's __attribute((annotate("count_use"))), function with
or without function body should be counted.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D111109

Patch by:  python3kgae (Xiang Li)