[flang] Fix a bug in the character runtime

The number of bytes copied in CopyAndPad should depend on the size of
the type being copied, not on its shift value (which in the case of char
is 0, leading to no bytes at all being copied).

Add unit tests for CharacterMin and CharacterMax, which exercise this
code path.

Differential Revision: https://reviews.llvm.org/D101355