[flang] Initial UTF-8 support in runtime I/O

Implements UTF-8 encoding and decoding for external units
with OPEN(ENCODING='UTF-8').  This encoding applies to default
CHARACTER values that are not 7-bit ASCII as well as to
the wide CHARACTER kinds 2 and 4.  Basic testing is in place
via direct calls to the runtime I/O APIs, but serious checkout
awaits lowering support of the wide CHARACTER kinds.

Differential Revision: https://reviews.llvm.org/D122038