[lit] llvm-lit.in: specify file encoding to UTF-8

This is needed because otherwise if source dir is at location whose path
contains non-ASCII character then python will complain about
SyntaxError.

SyntaxError: Non-ASCII character '\xc4' in file
/home/D?vis/libc++/src/build/bin/llvm-lit on line 16, but no encoding
declared; see http://python.org/dev/peps/pep-0263/ for details

Patch by davispuh

Differential Revision: https://reviews.llvm.org/D50201

llvm-svn: 351113