[flang] Add support for BLOCK construct

[flang] Add support for BLOCK construct

A `BLOCK` statement opens a new scope. It is different from other scopes
in that implicitly typed entities are defined in the enclosing non-block
scope, not immediately in the block. This means that `IMPORT` statements
can cause them to be hidden.

Check that blocks can't have `IMPLICIT` statements in them. It is simpler
for the parser not to deal with the different between a
specification-part and a block-specification-part.

Change `ImplicitRules` to have a parent that is consulted when there isn't
an answer in the current one. For an interface body that does not happen
but for all other nested scopes it does. This parent link eliminates the
need for the `implicitRules_` stack. Make `isImplicitNoneType_` and
`isImplicitNoneExternal_` optional: not set means look in parent.
Fixes flang-compiler/f18#71.

Remove `CurrNonTypeScope()` and put the logic in `Symbol::FindSymbol`.

Original-commit: flang-compiler/f18@a153a2ee33ff493df6d4fb5e20d99d302d76ab43
Reviewed-on: https://github.com/flang-compiler/f18/pull/169
Tree-same-pre-rewrite: false