[target] Change target create's behavior wrt loading dependent files.

When creating a target, lldb loads all dependent files (i.e. libs in
LC_LOAD_DYLIB for Mach-O). This can be confusing, especially when two
versions of the same library end up in the shared cache. It's possible
to change this behavior, by specifying  target create -d <target> these
dependents are not loaded.

This patch changes the default behavior to only load dependent files
only when the target is an executable. When creating a target for a
library, it is now no longer necessary to pass -d. The user can still
override this behavior by specifying the -d option to change this
behavior.

rdar://problem/43721382

Differential revision: https://reviews.llvm.org/D51934

llvm-svn: 342634