Added FixIt support to printf format string checking.

- Refactored LengthModifier to be a class.
- Added toString methods in all member classes of FormatSpecifier.
- FixIt suggestions keep user specified flags unless incorrect.

Limitations:
- The suggestions are not conversion specifier sensitive. For example, if we have a 'pad with zeroes' flag, and the correction is a string conversion specifier, we do not remove the flag. Clang will warn us on the next compilation.

A    test/Sema/format-strings-fixit.c
M    include/clang/Analysis/Analyses/PrintfFormatString.h
M    lib/Analysis/PrintfFormatString.cpp
M    lib/Sema/SemaChecking.cpp

llvm-svn: 105680