Assume UTF-8 in TextFileFormat::readFileUTF8 as fallback

If the contents is not detected as UTF-8/16/32 (TextFileFormat::detect)
and decoding with the given defaultCodec fails, still assume UTF-8.

This affects reading in files for code model indexing. Better read in the
file with the wrong encoding than not reading it at all.

Change-Id: Ibc10b7268ecbed4e7fd50336182f6e71c7d93f5b
Reviewed-by: Pawel Faron <pawelfaron87@wp.pl>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>