[llvm-gsymutil] Print one-time DWO file missing warning under --quiet flag

FileCheck test added
```
./bin/llvm-lit -sv llvm/test/tools/llvm-gsymutil/X86/dwo-warning.test
```

Manual test steps:

- Create binary with split-dwarf:
```
clang++ -g -gdwarf-4 -gsplit-dwarf main.cpp -o main_split
```

- Remane the dwo file to a different name so llvm-gsymutil can't find it
```
mv main_split-main.dwo main_split-main__.dwo
```

- Now run llvm-gsymutil conversion, it should print out warning with and without the `--quiet` flag
```
$ ./bin/llvm-gsymutil --convert=./main_split
Input file: ./main_split
Output file (x86_64): ./main_split.gsym
warning: Unable to retrieve DWO .debug_info section for main_split-main.dwo
Loaded 0 functions from DWARF.
Loaded 12 functions from symbol table.
Pruned 0 functions, ended with 12 total
```

```
$ ./bin/llvm-gsymutil --convert=./main_split --quiet
Input file: ./main_split
Output file (x86_64): ./main_split.gsym
warning: Unable to retrieve DWO .debug_info section for some object files. (Remove the --quiet flag for full output)
Pruned 0 functions, ended with 12 total
```

Author: kusmour

llvm/lib/DebugInfo/GSYM/DwarfTransformer.cpp

@@ -537,17 +537,27 @@ void DwarfTransformer::handleDie(raw_ostream *OS, CUInfo &CUI, DWARFDie Die) {
 
 Error DwarfTransformer::convert(uint32_t NumThreads, raw_ostream *OS) {
   size_t NumBefore = Gsym.getNumFunctionInfos();
+  std::once_flag flag;
   auto getDie = [&](DWARFUnit &DwarfUnit) -> DWARFDie {
     DWARFDie ReturnDie = DwarfUnit.getUnitDIE(false);
     if (DwarfUnit.getDWOId()) {
       DWARFUnit *DWOCU = DwarfUnit.getNonSkeletonUnitDIE(false).getDwarfUnit();
-      if (OS && !DWOCU->isDWOUnit()) {
-        std::string DWOName = dwarf::toString(
-            DwarfUnit.getUnitDIE().find(
-                {dwarf::DW_AT_dwo_name, dwarf::DW_AT_GNU_dwo_name}),
-            "");
-        *OS << "warning: Unable to retrieve DWO .debug_info section for "
-            << DWOName << "\n";
+      if (!DWOCU->isDWOUnit()) {
+        if (OS) {
+          std::string DWOName = dwarf::toString(
+              DwarfUnit.getUnitDIE().find(
+                  {dwarf::DW_AT_dwo_name, dwarf::DW_AT_GNU_dwo_name}),
+              "");
+          *OS << "warning: Unable to retrieve DWO .debug_info section for "
+              << DWOName << "\n";
+        } else {
+          std::call_once(flag, []() {
+            outs()
+                << "warning: Unable to retrieve DWO .debug_info section for "
+                   "some "
+                   "object files. (Remove the --quiet flag for full output)\n";
+          });
+        }
       } else {
         ReturnDie = DWOCU->getUnitDIE(false);
       }

llvm/test/tools/llvm-gsymutil/X86/Inputs/main.cpp

@@ -0,0 +1,13 @@
+#include <iostream>
+#include <unistd.h>
+
+void foo() {
+    std::cout << "This is foo" << std::endl;
+}
+
+int main() {
+    std::cout << "hello world" << std::endl;
+    foo();
+    std::cout << "after foo" << std::endl;
+    return 0;
+}

llvm/test/tools/llvm-gsymutil/X86/Inputs/main_split

@@ -0,0 +1,32 @@
+$ cat > main.cpp
+#include <iostream>
+#include <unistd.h>
+
+void foo() {
+    std::cout << "This is foo" << std::endl;
+}
+
+int main() {
+    std::cout << "hello world" << std::endl;
+    foo();
+    std::cout << "after foo" << std::endl;
+    return 0;
+}
+
+$ clang++ -g -gdwarf-4 -gsplit-dwarf main.cpp -o main_split
+$ mv main_split-main.dwo main_split-main__.dwo
+
+// RUN: llvm-gsymutil --convert=%p/Inputs/main_split | FileCheck %s --check-prefix=WARNING
+// RUN: llvm-gsymutil --convert=%p/Inputs/main_split --quiet 2>&1 | FileCheck %s --check-prefix=WARNING-QUIET
+
+// WARNING: Input file:  {{.*\/main_split}}
+// WARNING: Output file (x86_64): {{.*\.gsym}}
+// WARNING: warning: Unable to retrieve DWO .debug_info section for main_split-main.dwo
+// WARNING: Loaded 0 functions from DWARF.
+// WARNING: Loaded 12 functions from symbol table.
+// WARNING: Pruned 0 functions, ended with 12 total
+
+// WARNING-QUIET: Input file: {{.*\/main_split}}
+// WARNING-QUIET: Output file (x86_64): {{.*\.gsym}}
+// WARNING-QUIET: warning: Unable to retrieve DWO .debug_info section for some object files. (Remove the --quiet flag for full output)
+// WARNING-QUIET: Pruned 0 functions, ended with 12 total