revert Highlight references to use FindUsages instead of ...

...(GetFSharpSymbol + ParseAndCheckResults.GetSymbolAtLocation + ParseAndCheckResults.GetUsesOfSymbolInFile)

I tested the latest master and mentioned that Highlight usages became much slower on large files (`tc.fs`): instead of to be instant as before, it took ~3 seconds to highlight a symbol with heavy CPU load. 

So I reverted it back to use single call to `FindUsages` and everything has become OK. 

@dsyme Can you please review this change and comment? Should we revert file-scope Rename refactoring as well?