[flang] Responses to review comments.

[flang] Responses to review comments.

 - dosemantics05.f90: Added tests for ASSOCIATE, BLOCK and SELECT TYPE statements and changed the error messages.
 - check-do.cc: Changed things so that FindScope() is only called once when DoConcurrentVariableEnforce is instantiated.  I changed the error message.  I changed the type and name of CS to be an std::set and be called SymbolContainer.
 - resolve-names.cc: I changed the Pre() function for parser::Statement to add the source range of a statement to both the current scope and all of its parents.  This fixed a problem with finding the current scope based on the source position.

Original-commit: flang-compiler/f18@085b2c18f3b6393a25beaa760a9d88a8ebd483f5
Reviewed-on: https://github.com/flang-compiler/f18/pull/612