[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

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1868,6 +1868,7 @@ void ModuleBitcodeWriter::writeDIModule(const DIModule *N,
   for (auto &I : N->operands())
     Record.push_back(VE.getMetadataOrNullID(I));
   Record.push_back(N->getLineNo());
+  Record.push_back(N->getIsDecl());
 
   Stream.EmitRecord(bitc::METADATA_MODULE, Record, Abbrev);
   Record.clear();