[clangd] Fix not being able to attach a debugger on macOS

Clangd is often waiting for input on getline as it awaits requests. If the
getline is interrupted, it causes the system call (read) to fail and the EINTR
error to be set. This can be seen when attaching a debugger such as LLDB on
macOS. On macOS (and possibly other operating systems), this system call is not
restarted after interruption but on Linux it is restarted which is why
attaching a debugger does work correctly there.

The solution is to work around the non-restarting system call by checking the
errno for EINTR when the stream fails and try again. This should be safe on all
Unixish platforms.

See also http://bugs.llvm.org/show_bug.cgi?id=32149 for some background
discussion.

Patch by Marc-Andre Laperle!
Differential Revision: https://reviews.llvm.org/D30675

llvm-svn: 297779