Be consistent about using "const Twine &" for filenames.

On this file we had a mix of
* Twine
* const char *
* StringRef

The two that make sense are
* const Twine & (caller convenience)
* consc char * (that is what will eventually be passed to open.

Given that sys::fs::openFileForRead takes a "const Twine &", I picked that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219224 91177308-0d34-0410-b5e6-96231b3b80d8
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