[flang] Runtime: implement INDEX intrinsic function

Implement INDEX in the runtime, reusing some infrastructure
(with generalization and renaming as needed) put into place
for its cousins SCAN and VERIFY.

I did not implement full Boyer-Moore substring searching
for the forward case, but did accelerate some advancement on
mismatches.

I (re)implemented unit testing for INDEX in the new gtest
framework, combining it with the tests that have recently
been ported to gtest for SCAN and VERIFY.

Differential Revision: https://reviews.llvm.org/D98553