[flang] Fix looping on LEN type parameter usage

When a LEN type parameter of one PDT is being used as the value
of a LEN type parameter in another PDT, expression rewriting can
loop infinitely due to an incorrect assumption that the same PDT's
parameters are being referenced.

Fixes LLVM bug https://github.com/llvm/llvm-project/issues/63198

Differential Revision: https://reviews.llvm.org/D153465