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 <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>
Reviewed-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Reviewed-by: Shi Steven <4f7a0884debfd9139745b184e69c58dae2b3e6ce@intel.com>
Tested-by: Leif Lindholm <a613079f0455438b29e28a9c5b9eb3779a81a33a@linaro.org>
Reviewed-by: Leif Lindholm <a613079f0455438b29e28a9c5b9eb3779a81a33a@linaro.org>
Reviewed-by: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>

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