...in LIBO_INTERNAL_ONLY, __cplusplus, non-MSVC case.

It turns out that sal_Unicode happens to not be mangled into any symbols that
make up the stable URE interface, so (for LIBO_INTERNAL_ONLY, at least) we are
free to replace the typedef to sal_uInt16 with a typedef to any integral type
layout-compatible with that.  (sal_Unicode does appear in some symbols in sal's
PRIVATE_textenc.1 section, but that is private between the sal and sal_textenc
libraries, so changing those symbols does not require a change of SONAME.)
C++11 chart16_t is the obvious choice (and will ultimately allow using u"..."
to write literals of type array-of-sal_Unicode).  Reportedly, char16_t is
supported since GCC 4.4 and Clang 2.9 but will only be available in MSVC 2015.

For plain C, we continue to use sal_uInt16.  We could theoretically use C11
char16_t from <uchar.h>, but at least the Mac OS X 10.11 SDK still does not
offer that C11 header.

For MSVC, we continue to use wchar_t (which is actually unsigned short, due to
/Zc:wchar_t-) for now.  Potential options there include dropping /Zc:wchar_t-
and using true wchar_t, or using C++11 char16_t once support for MSVC 2013 is
dropped.

Some code needed to be adapted that was written in a way assuming that
sal_Unicode is unsigned short (which indicates that changing sal_Unicode for
non-LIBO_INTERNAL_ONLY would be an ABI change).  OUStringBuffer::append can now
differentiate between being called with sal_Unicode (to append a single
character) and erroneously being called with sal_uInt16 (intending to append a
number's textual representation, for which the sal_Int32 overload must be used
instead).  Bugs found are 379fe040 "Assume that
this code wants to append a number, not a character" and
dc148335 "Assume this wants to append the
numerical representation."

The GDB support for pretty-printing of sal_Unicode-related data in
solenv/gdb/libreoffice/sal.py can presumably be simplified now.

Change-Id: I445b3a80e65b7cb004d9e08b38bdc9ee93bc9401
Reviewed-on: https://gerrit.libreoffice.org/20036Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>