[flang] Small improvements to name resolution

Resolve names and do implicit typing in ArrayElement and LoopBounds.

Fix problem with creation of UseErrorDetails: when a conflict occurs,
record both the original USE and the new one.

Resolve the type name in TypeAttrSpec::Extends. Move CheckUseError to
DeclarationVisitor so that it can be used in ResolveDerivedType.

Report error on unresolved names. I.e. after name resolution, check each
parser::Name and verify we have a Symbol for it. This is on by default
now but it could be an option.

Original-commit: flang-compiler/f18@1c8cbc6e53cc7e1b6909bbedf5370e27fd11572f
Reviewed-on: https://github.com/flang-compiler/f18/pull/173
Tree-same-pre-rewrite: false