[MachineOutliner] Don't outline from functions with a section marking.

The program might have unusual expectations for functions; for example,
the Linux kernel's build system warns if it finds references from .text
to .init.data.

I'm not sure this is something we actually want to make any guarantees
about (there isn't any explicit rule that would disallow outlining
in this case), but we might want to be conservative anyway.

Differential Revision: https://reviews.llvm.org/D46091

llvm-svn: 331007