BaseTools: Update CLANG{35,38}_WARNING_OVERRIDES to ignore unused vars

BaseTools: Update CLANG{35,38}_WARNING_OVERRIDES to ignore unused vars

Building with the CLANG35 and CLANG38 toolset fails because of variables
which are set but not otherwise used in the RELEASE build.

GCC added -Wno-unused-but-set-variable back in 2016, and later added
-Wno-unused-const-variable. Add those to CLANG35_WARNING_OVERRIDES and
CLANG38_WARNING_OVERRIDES.

Signed-off-by: Rebecca Cran <quic_rcran@quicinc.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>