MSVC uses lit for STL testing, running both the libcxx tests and our "native" suite of tests which has feature requirements that are not parsed from the test content, but supplied externally. dac21fd2 broke us by ignoring the initial value of the requires property of tests and using only the result of the REQUIRES: parser.
(This is my first LIT change, apologies if I'm missing some protocol.)
I think this can always be just test.requires because test.requires is set to [] on construction. Thus you'd end up adding both the requires set programatically on the test and the parsed ones.