[flang] Fixes for RESHAPE()

I fixed an assert caused by passing an empty array as the source= argument to
RESHAPE().  In the process, I noticed that there were no tests for RESHAPE(),
so I wrote a test that covers all the description in 16.9.163.  In the process,
I made the error messages more consistent and descriptive.  I also changed the
test to see if a reference to an intrinsic function was a constant to say that
it is a constant if it's a refererence to an invalid intrinsic.  This avoids
emitting multiple messages for the same erroneous source.

Differential Revision: https://reviews.llvm.org/D84904