[docs] Minor doc tweaks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182324 91177308-0d34-0410-b5e6-96231b3b80d8
PR14606: Debug Info for namespace aliases/DW_TAG_imported_module

This resolves the last of the PR14606 failures in the GDB 7.5 test
suite by implementing an optional name field for
DW_TAG_imported_modules/DIImportedEntities and using that to implement
C++ namespace aliases (eg: "namespace X = Y;").

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182328 91177308-0d34-0410-b5e6-96231b3b80d8