[LLVM] Stringref API deprecation fixes

The `startswith`/`endswith` functions in `StringRef` API were [changed](https://reviews.llvm.org/D136030) to
`starts_with` and `ends_with` to be compatible with `std::string` and
the older APIs were deprecated and removed.

src/target/llvm/codegen_hexagon.cc

@@ -126,9 +126,16 @@ void CodeGenHexagon::InitTarget() {
   const auto hvx_length_feature = "+hvx-length";  // +hvx-length{64|128}b
   for (const std::string& f : llvm_target_->GetTargetFeatures()) {
     llvm::StringRef fs(f);
+#if TVM_LLVM_VERSION >= 180
+    if (!fs.starts_with(hvx_length_feature)) continue;
+
+    ICHECK(fs.ends_with("b")) << "malformed target feature: " << f;
+#else
     if (!fs.startswith(hvx_length_feature)) continue;
 
     ICHECK(fs.endswith("b")) << "malformed target feature: " << f;
+#endif
+
     int hvx_bytes = 0;
     size_t len_begin = std::strlen(hvx_length_feature);
     ICHECK(!fs.substr(len_begin, fs.size() - len_begin - 1).getAsInteger(10, hvx_bytes))
@@ -639,7 +646,11 @@ runtime::Module BuildHexagon(IRModule mod, Target target) {
   Map<String, String> extra_args;
   if (target->attrs.count("mcpu")) {
     std::string mcpu = Downcast<String>(target->attrs.at("mcpu"));
+#if TVM_LLVM_VERSION >= 180
+    ICHECK(llvm::StringRef(mcpu).starts_with("hexagon"))
+#else
     ICHECK(llvm::StringRef(mcpu).startswith("hexagon"))
+#endif
         << "unexpected -mcpu value in target:" << mcpu;
     extra_args.Set("hex_arch", llvm::StringRef(mcpu).drop_front(strlen("hexagon")).str());
   }

src/target/llvm/codegen_llvm.cc

@@ -372,7 +372,11 @@ std::unique_ptr<llvm::Module> CodeGenLLVM::Finish() {
 void CodeGenLLVM::HandleImport(const std::string& code) {
   llvm::StringRef code_str(code);
   std::unique_ptr<llvm::Module> mlib;
+#if TVM_LLVM_VERSION >= 180
+  if (code_str.ends_with(".ll") || code_str.ends_with(".bc")) {
+#else
   if (code_str.endswith(".ll") || code_str.endswith(".bc")) {
+#endif
     mlib = llvm_target_->GetInstance().LoadIR(code);
   } else {
     mlib = llvm_target_->GetInstance().ParseIR(code);

src/target/llvm/llvm_instance.cc

@@ -916,7 +916,11 @@ std::string LLVMTarget::GetTargetMetadata(const llvm::Module& module) {
   if (llvm::Metadata* tvm_target = module.getModuleFlag("tvm_target")) {
     auto* mdstr = llvm::cast<llvm::MDString>(tvm_target);
     llvm::StringRef meta = mdstr->getString();
+#if TVM_LLVM_VERSION >= 180
+    if (meta.starts_with("llvm")) {
+#else
     if (meta.startswith("llvm")) {
+#endif
       return meta.str();
     }
   }