[analyzer] Allow suppressing diagnostics reported within the 'std' namespace

This is controlled by the 'suppress-c++-stdlib' analyzer-config flag.
It is currently off by default.

This is more suppression than we'd like to do, since obviously there can
be user-caused issues within 'std', but it gives us the option to wield
a large hammer to suppress false positives the user likely can't work
around.

llvm-svn: 178513