[flang] More changes in response to review comments.

 - resolve-names.cc: I reworded the message when a name appears in a
   locality-spec when a name is used that cannot appear in a variable
   definition context.
 - tools.cc: I removed the unused functions ```IsValueDummy()``` and
   ```IsModifiable()```.  I made the function ```GetAssociatedVariable()```
   static.  I cleaned up the code in ```GetAssociationRoot()```.  I cleaned up
   the code in ```IsOrContainsEventOrLockComponent()```.  I added a TODO to
   ```WhyNotModifiable()``` and made some other improvements to it.
 - tools.h: Removed some deleted and unnecessary functions.
 - I fixed up a couple of tests related to the changes in error messages.

Original-commit: flang-compiler/f18@47da8ff9c8c131a6fc7a4a6f4a8fa89c8b582b71
Reviewed-on: https://github.com/flang-compiler/f18/pull/596