Better diagnostics for string initialization.

This commit improves Clang's diagnostics for string initialization.
Where it would previously say:

  /tmp/a.c:3:9: error: array initializer must be an initializer list
  wchar_t s[] = "Hi";
          ^
  /tmp/a.c:4:6: error: array initializer must be an initializer list or string literal
  char t[] = L"Hi";
       ^

It will now say

  /tmp/a.c:3:9: error: initializing wide char array with non-wide string literal
  wchar_t s[] = "Hi";
          ^
  /tmp/a.c:4:6: error: initializing char array with wide string literal
  char t[] = L"Hi";
       ^

As a bonus, it also fixes the fact that Clang would previously reject
this valid C11 code:

  char16_t s[] = u"hi";
  char32_t t[] = U"hi";

because it would only recognize the built-in types for char16_t and
char32_t, which do not exist in C.