Fix <uchar.h> in for C++11.

It turns out that in C++11, char16_t and char32_t are built-in types;
language keywords. Just fix this by putting traditional _*_T_DECLARED
blocks around the definitions. We'll just predefine these in
<sys/_types.h>.

This also opens up the possibility to define char16_t in other header
files, if ever needed (e.g. if we would gain a <ctype.h> for
char16_t/char32_t).
Fix some statical clang analyzer warnings.