Update inline builtin handling to honor gnu inline attribute

Per the GCC info page:

    If the function is declared 'extern', then this definition of the
    function is used only for inlining.  In no case is the function
    compiled as a standalone function, not even if you take its address
    explicitly.  Such an address becomes an external reference, as if
    you had only declared the function, and had not defined it.

Respect that behavior for inline builtins: keep the original definition, and
generate a copy of the declaration suffixed by '.inline' that's only referenced
in direct call.

This fixes holes in c3717b68.

Differential Revision: https://reviews.llvm.org/D111009