[LTO] Fix the issue of resetting the triple to default when it's empty (#157829)

The empty triple is passed to lookupTarget function and it's not set to
default one. This issue is exposed after changes in
#157591.

Author: fzou1

llvm/lib/LTO/LTOCodeGenerator.cpp

@@ -376,11 +376,11 @@ bool LTOCodeGenerator::determineTarget() {
     return true;
 
   TripleStr = MergedModule->getTargetTriple().str();
-  llvm::Triple Triple(TripleStr);
   if (TripleStr.empty()) {
     TripleStr = sys::getDefaultTargetTriple();
-    MergedModule->setTargetTriple(Triple);
+    MergedModule->setTargetTriple(Triple(TripleStr));
   }
+  llvm::Triple Triple(TripleStr);
 
   // create target machine from info for merged modules
   std::string ErrMsg;

llvm/test/LTO/empty-triple.ll

@@ -0,0 +1,12 @@
+; RUN: llvm-as < %s >%t1
+; RUN: llvm-lto -exported-symbol=main -filetype=asm -o - %t1  2>&1 | FileCheck %s
+
+; The test is to check the triple is set to default one when it's empty.
+; Otherwise, an error will be raised by llvm-lto.
+
+; CHECK-LABEL: main
+; CHECK-NOT: error
+define void @main() {
+entry:
+  ret void
+}