Warn about non-standard format strings (pr12017)

Warn about non-standard format strings (pr12017)

This adds the -Wformat-non-standard flag (off by default,
enabled by -pedantic), which warns about non-standard
things in format strings (such as the 'q' length modifier,
the 'S' conversion specifier, etc.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151154 91177308-0d34-0410-b5e6-96231b3b80d8