Support: Extract sys::fs::readNativeFileToEOF() from MemoryBuffer

Extract the `readNativeFile()` loop from
`MemoryBuffer::getMemoryBufferForStream()` into `readNativeFileToEOF()`
to allow reuse. The chunk size is configurable; the default of `4*4096`
is exposed as `sys::fs::DefaultReadChunkSize` to allow sizing of
SmallVectors.

There's somewhere I'd like to read a usually-small file without overhead
of a MemoryBuffer; extracting existing logic rather than duplicating it.

Differential Revision: https://reviews.llvm.org/D115397