[MachineOutliner][NFC] Make outlined functions have internal linkage

The linkage type on outlined functions was private before. This meant that if
you set a breakpoint in an outlined function, the debugger wouldn't be able to
give a sane name to the outlined function.

This commit changes the linkage type to internal and updates any tests that
relied on the prefixes on the names of outlined functions.
 

llvm-svn: 329116