[Sema] Fix null pointer dereference handleAlwaysInlineAttr.

It's possible for `getCalleeDecl()` to return a null pointer.

This was encountered by a user of our downstream compiler.

The case involved a DependentScopeDeclRefExpr.

Since this seems to only be for a warning diagnostic, I skipped
the diagnostic check if it returned null. But mabye there's a
different way to fix this.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D146089