[flang] Special handling for VOLATILE and ASYNCHRONOUS

The VOLATILE and ASYNCHRONOUS attributes are special in two ways:
- they can be applied to use-associated variables
- if they are applied to a host-associated variable in a block, that
  variable has the attribute only within the scope of the block

The latter is implemented by making a new `HostAssocDetails` symbol
within the block where the attribute can be set without affecting the
variable in the outer scope. This is similar to how the SHARED locality
spec is implemented.

Fixes flang-compiler/f18#649.

Original-commit: flang-compiler/f18@471aba451341a65ff6014bbe30cb2c64cf905da6
Reviewed-on: https://github.com/flang-compiler/f18/pull/655