Finally implement rewriting global initializers which use external functions

Finally implement rewriting global initializers which use external functions
by creating an internal wrapper function with same signature as the external
function, and use it instead of the "real" function.

The wrapper then calls the external function using the same JIT function
resolution API that has been used before for rewriting instructions, since the
wrapper has an explicit call instruction which we can rewrite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13054 91177308-0d34-0410-b5e6-96231b3b80d8