Revert of clang: Remove unused warning suppression flags. (patchset #1 id:1 of https...

Revert of clang: Remove unused warning suppression flags. (patchset #1 id:1 of https://codereview.chromium.org/784183002/)

Reason for revert:
Caused this warning:

..\..\third_party\mesa\src\src\glsl\ast_to_hir.cpp(4067,3) :  warning(clang): struct 'ir_variable' was previously declared as a class [-Wmismatched-tags]
         struct ir_variable *var = (ir_variable *)node;
         ^
..\..\third_party\mesa\src\src\glsl/ir.h(306,7) :  note(clang): previous use is here
class ir_variable : public ir_instruction {
      ^
..\..\third_party\mesa\src\src\glsl\ast_to_hir.cpp(4067,3) :  note(clang): did you mean class here?
         struct ir_variable *var = (ir_variable *)node;
         ^~~~~~
         class
1 warning generated.

Original issue's description:
> clang: Remove unused warning suppression flags.
>
> BUG=
> TBR=cpu
>
> Committed: https://crrev.com/dd843a59d5564ef7d5113151b5078601ab621177
> Cr-Commit-Position: refs/heads/master@{#307408}

TBR=hans@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/798493002

Cr-Commit-Position: refs/heads/master@{#307871}