[clang] [flang] [llvm] [CodeGen] Add a FunctionSplittingMode target option (PR #226575)

[Sriraman Tallam via cfe-commits]

tmsri wrote:

Approving this patch, do not push until others have reviews and stamped it.

This patch adds the ability to completely disable splitting even when the bbsections profile is present, something that wasn't possible previously.  The rest of the options look fine.  

https://github.com/llvm/llvm-project/pull/226575