[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.

clang/lib/Driver/Driver.cpp

@@ -4189,7 +4189,7 @@ void Driver::BuildActions(Compilation &C, DerivedArgList &Args,
         break;
       }
 
-      if (auto *IAA = dyn_cast<InstallAPIJobAction>(Current)) {
+      if (isa<InstallAPIJobAction>(Current)) {
         Current = nullptr;
         break;
       }

clang/lib/Sema/SemaDecl.cpp

@@ -2960,7 +2960,7 @@ static bool mergeDeclAttribute(Sema &S, NamedDecl *D,
         S.mergeHLSLNumThreadsAttr(D, *NT, NT->getX(), NT->getY(), NT->getZ());
   else if (const auto *SA = dyn_cast<HLSLShaderAttr>(Attr))
     NewAttr = S.mergeHLSLShaderAttr(D, *SA, SA->getType());
-  else if (const auto *SupA = dyn_cast<SuppressAttr>(Attr))
+  else if (isa<SuppressAttr>(Attr))
     // Do nothing. Each redeclaration should be suppressed separately.
     NewAttr = nullptr;
   else if (Attr->shouldInheritEvenIfAlreadyPresent() || !DeclHasAttr(D, Attr))