Fix unused-function warning differently from r220853

Fix unused-function warning differently from r220853

Rather than executing this code only needed for an assertion even in a
non-asserts build, just roll the function into the assert. The assertion
text literally describes the two cases so it doesn't seem like this
benefits much from having a separate function (& have to hassle about
ifndef NDEBUG it out, etc)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220970 91177308-0d34-0410-b5e6-96231b3b80d8