[clang] Add a command line flag for the Machine Function Splitter.

This patch adds a command line flag for the machine function splitter
(added in rG94faadaca4e1).

-fsplit-machine-functions
Split machine functions using profile information (x86 ELF). On
other targets an error is emitted. If profile information is not
provided a warning is emitted notifying the user that profile
information is required.

Differential Revision: https://reviews.llvm.org/D87047