NFC: fix range loop analysis warning. (#38747)

Silence warning by using reference type (`&`) to avoid creating a copy.

Author: dan-zheng

lib/IRGen/GenProto.cpp

@@ -3538,7 +3538,7 @@ llvm::Constant *IRGenModule::getAddrOfGenericEnvironment(
         llvm::SmallVector<uint16_t, 4> genericParamCounts;
         unsigned curDepth = 0;
         unsigned genericParamCount = 0;
-        for (const auto gp : signature.getGenericParams()) {
+        for (const auto &gp : signature.getGenericParams()) {
           if (curDepth != gp->getDepth()) {
             genericParamCounts.push_back(genericParamCount);
             curDepth = gp->getDepth();