Clean up stdin file reading of compiler.

Uses "stdinfile" as the module identifier now, which prevents problems
with - colliding with the minus operator. This fixes #2422 and #2702.

compiler/passes.nim

@@ -170,11 +170,7 @@ proc processModule(module: PSym, stream: PLLStream, rd: PRodReader) =
     openPasses(a, module)
     if stream == nil:
       let filename = fileIdx.toFullPathConsiderDirty
-      if module.name.s == "-":
-        module.name.s = "stdinfile"
-        s = llStreamOpen(stdin)
-      else:
-        s = llStreamOpen(filename, fmRead)
+      s = llStreamOpen(filename, fmRead)
       if s == nil:
         rawMessage(errCannotOpenFile, filename)
         return