Use character literals for vexing initialization fixit hints.

Instead of suggesting " = 0" for "char c();", suggest " = '\0'", and similarly
for other char types (wide, 16, and 32). Add tests for all these, and since
this means testing such hints under C++0x, add tests for some untested C++0x
hint cases in the existing code, including suggesting nullptr for pointer
initialization.

This sets up the initialization helper to provide better type fidelity that
will be especially helpful for non-assignment cases (such as fixit-correcting
NULL usage in function calls (eg: foo(char) + foo(NULL) => foo('\0') instead
of the less informative foo(0)))

llvm-svn: 153008