Add hook to add attributes to function declarations that we know

Add hook to add attributes to function declarations that we know
about, whether they are builtins or not. Use this to add the
appropriate "format" attribute to NSLog, NSLogv, asprintf, and
vasprintf, and to translate builtin attributes (from Builtins.def)
into actual attributes on the function declaration.

Use the "printf" format attribute on function declarations to
determine whether we should do format string checking, rather than
looking at an ad hoc list of builtins and "known" function names.

Be a bit more careful about when we consider a function a "builtin" in
C++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64561 91177308-0d34-0410-b5e6-96231b3b80d8