InlineTrans for routines with optional arguments: Handling non-trivial cases of PRESENT(...) in if-branches

[schreiberx]

PR #2801 will introduce some preliminary support of optional arguments.

This only covers simple cases such as

if (PRESENT(var)) then
...
else
...
end if

where the else-branch can be dropped.

Other cases using different conditionals not directly based on PRESENT(...) are not yet supported.
E.g. if (PRESENT(var) .and. PRESENT(var2)).