libc++ compile-only ".compile.pass.cpp" tests

[CaseyCarter]

libc++ uses the filename suffix .compile.pass.cpp to indicate a source file that must compile, but need not link nor be run. Our test runner doesn't understand this distinction and treats these tests like all other .pass.cpp tests which must run and return 0 status. When such a test doesn't have a main function, we have a problem.

Ideally we should teach our test runner about libc++'s compile-only convention, and perhaps expand that coverage into std which has int main() {} // COMPILE-ONLY tests which we waste time linking and running.