[c++20] Further extend the set of comparisons broken by C++20 that we

accept as an extension.

This attempts to accept the same cases a GCC, plus cases where a
comparison is rewritten to an operator== with an integral but non-bool
return type; this is sufficient to avoid most problems with various
major open-source projects (such as ICU) and appears to fix all but one
of the comparison-related C++20 build breaks in LLVM.

This approach is being pursued for standardization.