diagnostics: Fix mojibake from displaying UTF-8 on Windows consoles

diagnostics: Fix mojibake from displaying UTF-8 on Windows consoles

UTF-8 characters in diagnostic output (such as the warning emoji ⚠️
used by fanalyzer) display as mojibake on Windows unless the utf8
code page is being used

This patch adds UTF-8 to UTF-16 conversion when outputting to a console
on Windows.

gcc/ChangeLog:
	* pretty-print.cc (decode_utf8_char): Move forward declaration.
	(mingw_utf8_str_to_utf16_str): New function to convert UTF-8 to UTF-16.
	(is_console_handle): New function to detect Windows console handles.
	(write_all): Add UTF-8 to UTF-16 conversion for console output,
	falling back to WriteFile for ASCII strings and regular files.

Signed-off-by: Peter Damianov <peter0x44@disroot.org>
Signed-off-by: Jonathan Yong <10walls@gmail.com>