[clang][CodeGen] Fix gcc warning about unused variable [NFC]

Without the fix gcc warned with
 ../../clang/lib/CodeGen/CGBuiltin.cpp:1022:19: warning: unused variable 'DRE' [-Wunused-variable]
  1022 |   if (const auto *DRE = dyn_cast<DeclRefExpr>(Base)) {
       |                   ^~~

Fix the warning by removing the unused variable and change the "dyn_cast"
to "isa".