Summary:
Given that mangling now respects `extern "C" {}` declarations, pnames of C function will have no mangling and we don't need to discard mangled part from procname.
Move `malloc` detection to `get_builtin_pname_opt` function (together with all others)
Reviewed By: dulmarod
Differential Revision: D3804402
fbshipit-source-id: 9ae9991