[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).
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