BaseTools/tools_def CLANG38: add -Wno-unused-const-variable

BaseTools/tools_def CLANG38: add -Wno-unused-const-variable

Commit 8b6366f87584 ("BaseTools/GCC: set -Wno-unused-const-variable
on RELEASE builds") suppresses warnings about unused constant
variables in RELEASE builds when building with GCC, given that they
break the build under our warnings-as-errors policy.

Do the same for CLANG38.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=790
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Shi Steven <steven.shi@intel.com>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>