[lldb/Python] Fix the infinitely looping Python prompt bug

Executing commands below will get you bombarded by a wall of Python
command prompts (>>> ).

$ echo 'foo' | ./bin/lldb -o script
$ cat /tmp/script
script
print("foo")
$ lldb --source /tmp/script

The issue is that our custom input reader doesn't handle EOF. According
to the Python documentation, file.readline always includes a trailing
newline character unless the file ends with an incomplete line. An empty
string signals EOF. This patch raises an EOFError when that happens.

[1] https://docs.python.org/2/library/stdtypes.html#file.readline

Differential revision: https://reviews.llvm.org/D81898