[flang] [llvm] [flang-rt] Add PDT LEN parameter support: Concrete type cache (PR #181008)

[Peter Klausler via llvm-commits]

klausler wrote:

So the `concreteType` in the cache is a pointer to a `DerivedType` description that has been created at run time with offsets and sizes and bounds and lengths and derived type parameter values specialized for a particular collection of LEN PDT values?

https://github.com/llvm/llvm-project/pull/181008