Printf format strings: Added some more tests and fixed some minor bugs.

- Precision toStrings shouldn't print a dot when they have no value.
- Length of char length modifier is now returned correctly.
- Added several fixit tests.

Note: fixit tests are currently broken due to a bug in HighlightRange. Marking as XFAIL for now.

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

llvm-svn: 106275