We run the tests for -Wthread-safety-{negative,verbose} with the new
attributes as well as the old ones. Also put the macros in a header so
that we don't have to copy them all around.
The warn-thread-safety-parsing.cpp test checks for warnings depending on
the actual attribute name, so it can't undergo the same treatment.
Together with D49275 this should fix PR33754.
Can you add another newline above this comment, to fit my sense of aesthetics? :-P