[LLD][COFF] Fix pdb loading when the path points to a removable device

[aganea]

Summary

As described here, this patch fixes the case where an OBJ has a reference to a PDB path which happens to map locally to a removable device, drive which has no media inserted at the time of the link. In that case, the first attempt to load the PDB returns "resource unavailable try again". This prevents retrying the PDB load with the OBJ's path.

[zturner]

Lgtm

COFF/PDB.cpp

if (auto Err = mergeTypeAndIdRecords(IDTable, TypeTable,
       fatal("codeview::mergeTypeAndIdRecords failed: " +
             toString(std::move(Err)));
   }
   return ObjectIndexMap;
 }
 
 static Expected<std::unique_ptr<pdb::NativeSession>>
 tryToLoadPDB(const GUID &GuidFromObj, StringRef TSPath) {
+  // Ensure the file exists before anything else. We want to return ENOENT,
+  // "file not found", even if the path points to a removable device (in which
+  // case the return message would be EAGAIN, "resource unavailable try again")
+  if (!llvm::sys::fs::exists(TSPath))
+    return errorCodeToError(std::error_code(ENOENT, std::generic_category()));
+
   ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr = MemoryBuffer::getFile(
       TSPath, /*FileSize=*/-1, /*RequiresNullTerminator=*/false);
   if (!MBOrErr)
     return errorCodeToError(MBOrErr.getError());
 
   std::unique_ptr<pdb::IPDBSession> ThisSession;
   if (auto EC = pdb::NativeSession::createFromPdb(
           MemoryBuffer::getMemBuffer(Driver->takeBuffer(std::move(*MBOrErr)),