[DebugInfo] Support Fortran 'use <external module>' statement.
The main change is to add a 'IsDecl' field to DIModule so
that when IsDecl is set to true, the debug info entry generated
for the module would be marked as a declaration. That way, the debugger
would look up the definition of the module in the gloabl scope.
Please see the comments in llvm/test/DebugInfo/X86/dimodule.ll
for what the debug info entries would look like.
Differential Revision: https://reviews.llvm.org/D93462