Correctly initialize the DW_AT_comp_dir attribute of Clang module skeleton CUs

Before this patch a Clang module skeleton CU would have a
DW_AT_comp_dir pointing to the directory of the module map file, and
this information was not used by anyone. Even worse, LLDB actually
resolves relative DWO paths by appending it to DW_AT_comp_dir. This
patch sets it to the same directory that is used as the main CU's
compilation directory, which would make the LLDB code work.

Differential Revision: https://reviews.llvm.org/D76377