Remove the IsVolatileSize parameter of getOpenFileSlice.

getOpenFileSlice gets passed the map size, so it makes no sense to say that
the size is volatile. The code will not even compute the size.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219226 91177308-0d34-0410-b5e6-96231b3b80d8