Fix clang-format vim integration issue with non-ascii characters

clang-format.py currently seems to treat vim.current.buf as ascii-encoded data,
which leads to an UnicodeDecodeError when trying to format any text containing
non-ascii characters:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../tools/clang/tools/clang-format/clang-format.py", line 110, in <module>
    main()
  File ".../tools/clang/tools/clang-format/clang-format.py", line 87, in main
    stdout, stderr = p.communicate(input=text.encode(encoding))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 3996: ordinal not in range(128)

llvm-svn: 284988