[flang][OpenMP] Avoid analyzing assumed-size array bases (#150324)
A check for character substrings masquerading as array sections was
using expression analyzer on the array base. When this array happened to
be an assumed-size array, the analyzer emitted a semantic error that did
not correspond to any issue with the source code.
To avoid that, check whether the object is an assumed-size array before
using the expression analyzer on it.
While at it, replace the call to GetShape with a simple check for rank,
since that's the only information needed.
Fixes https://github.com/llvm/llvm-project/issues/150297