Sink DwarfUnit::constructImportedEntityDIE into DwarfCompileUnit.

So that it has access to getOrCreateGlobalVariableDIE. If we ever support
decsribing using directive in C++ classes (thus requiring support in type
units), it will certainly use another mechanism anyway.

Differential Revision: http://reviews.llvm.org/D5975

llvm-svn: 220594