[flang] Fix a crash when a BOZ literal is used as a relational operator

Summary:
Expressions like `iVar==z'fe'` were causing an assertion error  because
the `Relate()` function in `Evaluate/tools.cpp` that processes
relational operators didn't deal with BOZ literals, which are typeless.
I fixed this by checking to see if the operands are BOZ literals.  If
so, if the other operand is REAL, I convert them to REAL.  Otherwise, I convert
them to integers with default kind.

I also added a test to resolve63.f90 that triggers the problem.

Reviewers: tskeith, DavidTruby

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83917