[flang] Fix calls to LBOUND() intrinsic for arrays with lower bounds not 1

Constant folding for calls to LBOUND() was not working when the lower bound of
a constant array was not 1.

I fixed this and re-enabled the test in Evaluate/folding16.f90 that previously
was silently failing.  I slightly changed the test to parenthesize the first
argument to exercise all of the new code.

Differential Revision: https://reviews.llvm.org/D95894