[InstrProf] Don't take the address of alwaysinline available_externally functions

Doing so breaks compilation of the following C program
(under -fprofile-instr-generate):

 __attribute__((always_inline)) inline int foo() { return 0; }

 int main() { return foo(); }

At link time, we fail because taking the address of an
available_externally function creates an undefined external reference,
which the TU cannot provide.

Emitting the function definition into the object file at all appears to
be a violation of the langref: "Globals with 'available_externally'
linkage are never emitted into the object file corresponding to the LLVM
module."

Differential Revision: https://reviews.llvm.org/D34134

llvm-svn: 305327