[libc++][test] Skip string_view tests for other vendors on older modes

`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