The next step along the way to getting good error messages for bad archives.

As mentioned in commit log for r276686 this next step is adding a new
method in the ArchiveMemberHeader class to get the full name that
does proper error checking, and can be use for error messages.

To do this the name of ArchiveMemberHeader::getName() is changed to
ArchiveMemberHeader::getRawName() to be consistent with
Archive::Child::getRawName().  Then the “new” method is the addition
of a new implementation of ArchiveMemberHeader::getName() which gets
the full name and provides proper error checking.  Which is mostly a rewrite
of what was Archive::Child::getName() and cleaning up incorrect uses of
llvm_unreachable() in the code which were actually just cases of errors
in the input Archives.

Then Archive::Child::getName() is changed to return Expected<> and use
the new implementation of ArchiveMemberHeader::getName() .

Also needed to change Archive::getMemoryBufferRef() with these
changes to return Expected<> as well to propagate Errors up.
As well as changing Archive::isThinMember() to return Expected<> .

llvm-svn: 277177