MC: Don't access the filesystem in MCContext's constructor

MCContext shouldn't be accessing the filesystem - that's a gross
layering violation and makes it awkward to use as a library or in a
daemon where it may not even be allowed filesystem access.

The CWD lookup here is normally redundant anyway, since the calling
context either also looks up the CWD or sets this to something more
specific. Here, we fix up the one caller that doesn't already set up a
debug compilation dir and make it clear that the responsibility for
such set up is in the users of MCContext.

llvm-svn: 264109