Add writeFileWithSystemEncoding to LibLLVMSuppor.

This patch adds to LLVMSupport the capability of writing files with
international characters encoded in the current system encoding. This
is relevant for Windows, where we can either use UTF16 or the current
code page (the legacy Windows international characters). On UNIX, the
file is always saved in UTF8.

This will be used in a patch for clang to thoroughly support response
files creation when calling other tools, addressing PR15171. On
Windows, to correctly support internationalization, we need the
ability to write response files both in UTF16 or the current code
page, depending on the tool we will call. GCC for mingw, for instance,
requires files to be encoded in the current code page. MSVC tools
requires files to be encoded in UTF16.

Patch by Rafael Auler!