[flang] Forward references to COMMON from specification expr under IMPLICIT NONE

As a near-universal extension, Fortran compilers permit forward references
to dummy arguments and variables in COMMON blocks from specification expressions
before an explicit type-declaration-stmt appears for those variables
under IMPLICIT NONE, so long as those variables are later explicitly typed
with the types that regular implicit typing rules would have given them
(usually default INTEGER).

F18 implemented this extension for dummy arguments, but not variables in
COMMON blocks.  Extend the extension to also accept variables in COMMON.

Differential Revision: https://reviews.llvm.org/D145743