Reduce Driver verbosity by suppressing unused warning about another

cluster of -W options.

The list: -Waggregate-return -Wbad-function-cast -Wcast-qual
-Wformat-security -Wformat=2 -Wformat -Wmissing-declarations
-Wnewline-eof -Wno-parentheses -Wpacked -Wredundant-decls -Wshadow
-Wsign-compare -Wstrict-overflow= -Wstrict-overflow -Wuninitialized
-Wunknown-pragmas.

llvm-svn: 68402