[flang] Enabling automatic declaration of names in SHARED locality-spec's

Prior to this change, the compiler issued an error message when a name in a
SHARED locality-spec had not been declared explicitly in an enclosing scope.
Presumably, this was done to enforce constraint C1124.  This constraint states
that "A variable-name in a locality-spec shall be the name of a variable in the
innermost executable construct or scoping unit that includes the DO CONCURRENT
statement".  For LOCAL and LOCAL_INIT locality-spec's, we were automatically
creating a local variable in the situation where one had not been explicitly
declared rather than issuing an error message.

The only compiler I could find that implements the SHARED locality-spec was the
PGI compiler, which automatically creates a variable and does not emit an error
message.  Also, @vdonaldson said that he had consulted with a member of the
Fortran standards committee who said that the correct thing was to create a
variable if necessary.

This change has the compiler creating a variable in the enclosing scope of a DO
CONCURRENT statement if necessary.  I also changed a test to adapt to the new
behavior.  I also consolidated the semantic checking for the constraints
associated with all of the locality-spec's.

Original-commit: flang-compiler/f18@d9253a76fcbc49df11c017557b365dfeed7c2b3b
Reviewed-on: https://github.com/flang-compiler/f18/pull/551
Tree-same-pre-rewrite: false