devel/cdecl: Upgrade to 11.15

ChangeLog:
Atomic arrays
_Atomic arrays are now correctly forbidden:

typedef int A[2]
explain _Atomic A x // error: _Atomic array of T is illegal
Note that:

_Atomic int x[2]; // OK: array of _Atomic T is legal
is legal.

CDECLRC environment variable
If none of the --config, -c, --no-config, nor -C command line options
are given and the value of the CDECLRC environment variable isn't empty,
it's used as the full path to the cdeclrc file to read upon start-up.

Configuration file default language
Previously, cdecl would temporarily set the current language to the
latest supported version of C++ while reading a configuration file. It
no longer does this as a special case. Now, as always, it sets the
default language based on the executable name.

enum alignment in C11
Alignment of enums is now correctly allowed in C11.

Types as objects
Previously declared types are now checked to ensure they're not being
used as objects:

cdecl> struct S
cdecl> explain int S // error (now)