[clang] Use IsVolatile=true and RequiresNullTerminator=false for PCMs

This change got missed while upstreaming
https://reviews.llvm.org/D77772. This is the part of that change that
actually passes the correct arguments when opening a PCM.

The test didn't catch this because it starts at the
`MemoryBuffer::getOpenFile` level. It's not really possible to test
`ModuleManager::addModule` itself to verify how the file was opened.