[flang] Define & implement a lowering support API IsContiguous() in runtime

Create a new flang/runtime/support.cpp module to hold miscellaneous
runtime APIs to support lowering, and define an API IsContiguous() to
wrap the member function predicate Descriptor::IsContiguous().
And do a little clean-up of other API headers that don't need to expose
Runtime/descriptor.h.

Differential Revision: https://reviews.llvm.org/D114752