[flang] Check conflicting construct names

Check that construct names don't conflict with other construct names or
entities in the same scoping unit.

This requires adding symbols for them. MiscDetails was added for cases
like this where we don't need to save much information about the symbol.
References to construct names are recorded too, so those names are now
expected to be resolved and no longer are exceptions in rewrite-parse-tree.cc

Checking that construct names match is still done in resolve-labels.cc.

Original-commit: flang-compiler/f18@0ad79ff029f6f3001778e2fd6f00153906effec8
Reviewed-on: https://github.com/flang-compiler/f18/pull/210
Tree-same-pre-rewrite: false