ConnectionFileDescriptor::BytesAvailable reading too many command bytes

ConnectionFileDescriptor::BytesAvailable was reading multiple command
bytes from the command pipe but only processing the first. This
change only allows one byte to be read at a time, ensuring that all
get handled.

This isn't known to cause any bugs, but it might cause current/future
bugs.

Differential Revision: http://reviews.llvm.org/D9098

llvm-svn: 235322