[flang] resolve-names.cc reorganization and cleanup.

[flang] resolve-names.cc reorganization and cleanup.

Add ScopeHandle class to manage the stack of scopes and MakeSymbol(), which
makes a symbol in the current scope. Move the call to ApplyImplicitScopes()
into PopScope() as it must be done for each scope.

Add ModuleVisitor class to manage module-related resolution. Currently that
consists of beginning and ending modules and access statements. This is
preparation for further module work.

Replace references to parser::CharBlock with SourceName.

Original-commit: flang-compiler/f18@913df85e48f1e06d0c75bb517f5edd898e8efd69
Reviewed-on: https://github.com/flang-compiler/f18/pull/79
Tree-same-pre-rewrite: false