[Clang][Driver][LTO] Fix empty stats filename when in LTO mode (#71197)

Previously, if a linker argument (i.e. -Wl) is presented before any input
filenames, Gnu driver would use the InputInfo object of that argument to generate stats filename for LTO backend, causing an empty filename. This patch fixes such issue.

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -535,7 +535,15 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   if (D.isUsingLTO()) {
     assert(!Inputs.empty() && "Must have at least one input.");
-    addLTOOptions(ToolChain, Args, CmdArgs, Output, Inputs[0],
+    // Find the first filename InputInfo object.
+    auto Input = llvm::find_if(
+        Inputs, [](const InputInfo &II) -> bool { return II.isFilename(); });
+    if (Input == Inputs.end())
+      // For a very rare case, all of the inputs to the linker are
+      // InputArg. If that happens, just use the first InputInfo.
+      Input = Inputs.begin();
+
+    addLTOOptions(ToolChain, Args, CmdArgs, Output, *Input,
                   D.getLTOMode() == LTOK_Thin);
   }
 

clang/test/Driver/save-stats.c

@@ -20,6 +20,8 @@
 // CHECK-INVALID: invalid value 'bla' in '-save-stats=bla'
 
 // RUN: %clang -target x86_64-linux-unknown -save-stats -flto -o obj/dir/save-stats.exe %s -### 2>&1 | FileCheck %s -check-prefix=CHECK-LTO
+// Previously `-plugin-opt=stats-file` would use empty filename if a linker flag (i.e. -Wl) is presented before any input filename.
+// RUN: %clang --target=x86_64-linux-unknown -save-stats -flto -o obj/dir/save-stats.exe -Wl,-plugin-opt=-dummy %s -### 2>&1 | FileCheck %s -check-prefix=CHECK-LTO
 // CHECK-LTO: "-stats-file=save-stats.stats"
 // CHECK-LTO: "-o" "obj/dir{{/|\\\\}}save-stats.exe"
 // CHECK-LTO: "-plugin-opt=stats-file=save-stats.stats"