[Patch, Fortran] PR35203 - Fix VALUE + OPTIONAL handling for pass-by-value arguments

[Tobias Burnus]

Dear all,

using VALUE, gfortran passes the arguments by value.* That works well, 
except if VALUE is combined with OPTIONAL. Currently, "call foo(0)" and 
"call foo()" are indistinguishable.

With this patch, a hidden argument is added which includes the present 
information. I think that's the least intrusive version which also has 
the performance advantage of continuing to use pass-by-value semantics.

Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias

PS: At some point, we need to handle VALUE with arrays, derived types 
and class. For those, one should pass by reference, doing a copy in. In 
that case, using the NULL-pointer check for present() should work. (PR 
49802)

* Except for character. (Note: value+optional for characters currently 
fails with an ICE, also tracked at PR 49802.)
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