[NFC] Turn "load dependent files" boolean into an enum

This is an NFC commit to refactor the "load dependent files" parameter
from a boolean to an enum value. We want to be able to specify a
default, in which case we decide whether or not to load the dependent
files based on whether the target is an executable or not (i.e. a
dylib).

This is a dependency for D51934.

Differential revision: https://reviews.llvm.org/D51859

llvm-svn: 342633