[Driver][CodeGen] Add -fpatchable-function-entry=N[,0]

In the backend, this feature is implemented with the function attribute
"patchable-function-entry". Both the attribute and XRay use
TargetOpcode::PATCHABLE_FUNCTION_ENTER, so the two features are
incompatible.

Reviewed By: ostannard, MaskRay

Differential Revision: https://reviews.llvm.org/D72222