Rename DIImportedModule to DIImportedEntity and allow imported declarations

DIBuilder::createImportedDeclaration isn't fully plumbed through (note,
lacking in AsmPrinter/DwarfDebug support) but this seemed like a
sufficiently useful division of code to make the subsequent patch(es)
easier to follow.
Fix crash on invalid in template type diffing.

This is a fix for PR15895, where Clang will crash when trying to print a
template diff and the template uses an address of operator.  This resulted
from expecting a DeclRefExpr when the Expr could have also been
UnaryOperator->DeclRefExpr.