Added support for additional format string checking for the printf

family of functions.  Previous functionality only included checking to
see if the format string was a string literal.  Now we check parse the
format string (if it is a literal) and perform the following checks:

(1) Warn if: number conversions (e.g. "%d") != number data arguments.

(2) Warn about missing format strings  (e.g., "printf()").

(3) Warn if the format string is not a string literal.

(4) Warn about the use se of '%n' conversion.  This conversion is
    discouraged for security reasons.

(5) Warn about malformed conversions.  For example '%;', '%v'; these
    are not valid.

(6) Warn about empty format strings; e.g. printf("").  Although these
    can be optimized away by the compiler, they can be indicative of
    broken programmer logic.  We may need to add additional support to
    see when such cases occur within macro expansion to avoid false
    positives.

(7) Warn if the string literal is wide; e.g. L"%d".

(8) Warn if we detect a '\0' character WITHIN the format string.

Test cases are included.

llvm-svn: 41076