[clang] Fix two gcc warnings about unused variables [NFC]

Without the fix gcc warns like
 ../../clang/lib/Sema/SemaDecl.cpp:2963:24: warning: unused variable 'SupA' [-Wunused-variable]
  2963 |   else if (const auto *SupA = dyn_cast<SuppressAttr>(Attr))
       |                        ^~~~
and
 ../../clang/lib/Driver/Driver.cpp:4192:17: warning: unused variable 'IAA' [-Wunused-variable]
  4192 |       if (auto *IAA = dyn_cast<InstallAPIJobAction>(Current)) {
       |                 ^~~

Remove the unused variables and change the "dyn_cast"s into "isa"s.