`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
Even if these comments have a benefit in .h files (for editors that
care about language but can't be configured to treat .h as C++ code),
they certainly have no benefit for files with the .cpp extension.
Discussed in D110794.