[test/ubsan] Allow unused FileCheck prefixes in TypeCheck/vptr.cpp

This test started failing after https://reviews.llvm.org/D95849
defaulted --allow-unused-prefixes to false.

Taking a look at the test, I didn't see an obvious need to add
OS-specific check lines for each supported value of %os.

rdar://74207657

GitOrigin-RevId: 897bec937e8156061d0546c4a4bf925fdcdb672c
1 file changed