Always write the session file in UTF-8.

Always write the session file in UTF-8.

This patch attempts to solve the Python 2 / Python 3 incompatibilities by
introducing a new `encoded_file` abstraction that we use instead of
`io.open()`.  The problem with the builtin implementation of `io.open` is
that `read` and `write` accept and return `unicode` objects, which are not
always convenient to work with in Python 2.  We solve this by making
`encoded_file.open()` return the same object returned by `io.open()` but
with hooked `read()` and `write()` methods.  These hooked methods will
accept binary or text data, and conditionally convert what it gets to a
`unicode` object using the correct encoding.  When calling `read()` it
also does any conversion necessary to convert the output back into the
native `string` type of the running python version.

Differential Revision: http://reviews.llvm.org/D16736

llvm-svn: 259379