DebugInfo: Rename CreatePointerType to getOrCreateTypeDeclaration

To make this more consistent with 'getOrCreateType' & clarify the
distinction between the two. The only thing I couldn't quite communicate
in the name is that getOrCreateTypeDeclaration may actually produce a
full definition (in -fno-limit-debug-info) but the point is to call it
whenever only a declaration is needed & the implementation can choose
whether to provide a declaration or definition.

(also, unfortunately, getOrCreateType can produce declarations too - we
should sure this up by making it not do that - any caller that can
tolerate a declaration should be calling getOrCreateTypeDeclaration
instead)