[flang] Fix crash from a blank BIND(C,NAME="") on subprogram

A recent change fixed the processing of BIND(C,NAME=expr) character
expressions so that they are evaluated as constants in the scope of
the subprogram.  However, when the character name expression results
in an empty value after trimming, the compiler emits a warning message,
and this message is now causing a crash due to a lack of statement
context.  To fix, extend the deferred processing of the BIND(C,NAME="")
so that a basic statement context exists.

Differential Revision: https://reviews.llvm.org/D126416