`string_view` is supported all the way back to C++03 as an extension in
`libc++`, and so many of the tests run in all standards modes for all vendors.
This is unlikely desired by other standard library vendors using our test suite.
So, disable the tests for vendors other than `libc++` in these older standards
modes.
Differential Revision: https://reviews.llvm.org/D126850
Some tests in `string.view.comparison` are not enabled due to previous lack of
support for `constexpr std::string`. Now that it is implemented, we can enable
these tests.
Differential Revision: https://reviews.llvm.org/D126737
