[llvm][dsymutil] Add DW_TAG_imported_declaration to accelerator table

**Summary**

After this patch, `dsymutil` will preserve `DW_TAG_imported_declarations`
entries in accelerator tables.

This allows consumers to resolve imported declarations even on
executables processsed through dsymutil.

This helps consumers, particularly LLDB's expression evaluator,
to resolve imported declarations (i.e., useful for namespace aliases
in C++) more efficiently.

**Testing**

* Added unit-test

Differential Revision: https://reviews.llvm.org/D143458