[lldb] [Host/ConnectionFileDescriptor] Do not use non-blocking mode

Disable non-blocking mode that's enabled only for file:// and serial://
protocols.  All read operations should be going through the select(2)
in ConnectionFileDescriptor::BytesAvaliable, which effectively erases
(non-)blocking mode differences in reading.  We do want to perform
writes in the blocking mode.

Differential Revision: https://reviews.llvm.org/D112442