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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181364 91177308-0d34-0410-b5e6-96231b3b80d8