This renames the neon_* tests to be more consistent with the original files that they were forked from. This makes it easier for maintainers to see which test files may need to be ported to MSan.
It also fixes the header comment for llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-vcvt.ll (instrumentation was updated in #126136 aka e9e6ba6a5e, but the comment was not).