[flang] Dodge bogus uninitialized data warning from gcc 10.1 via code cleanup

G++ 10.1 emits inappropriate "use of uninitialized data" warnings when
compiling f18.  The warnings stem from two sites in templatized code
whose multiple instantiations magnified the number of warnings.

These changes dodge those warnings by making some innocuous changes to
the code.  In the parser, the idiom defaulted(cut >> x), which yields a
parser that always succeeds, has been replaced with a new equivalent
pass<T>() parser that returns a default-constructed value T{} in an
arguably more readable fashion.  This idiom was the only attestation of
the basic parser cut, so it has been removed and the remaining code
simplified.  In Evaluate/traverse.h, a return {}; was replaced with a
return of a default-constructed member.

Differential Revision: https://reviews.llvm.org/D81747