kbuild: Remove unnecessary -Wno-unused-value

This flag turns off several other warnings that would
be useful. Most notably -warn_unused_result is disabled.
All of the following warnings are currently disabled:

UnusedValue
|-UnusedComparison
  |-warn_unused_comparison
|-UnusedResult
  |-warn_unused_result
|-UnevaluatedExpression
  |-PotentiallyEvaluatedExpression
    |-warn_side_effects_typeid
  |-warn_side_effects_unevaluated_context
|-warn_unused_expr
|-warn_unused_voidptr
|-warn_unused_container_subscript_expr
|-warn_unused_call

With this flag removed there are ~10 warnings.
Patches have been submitted for each of these warnings.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: clang-built-linux@googlegroups.com
Link: ClangBuiltLinux/linux#520
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.extrawarn

@@ -67,7 +67,6 @@ else
 
 ifdef CONFIG_CC_IS_CLANG
 KBUILD_CFLAGS += -Wno-initializer-overrides
-KBUILD_CFLAGS += -Wno-unused-value
 KBUILD_CFLAGS += -Wno-format
 KBUILD_CFLAGS += -Wno-sign-compare
 KBUILD_CFLAGS += -Wno-format-zero-length