[lldb][unittest] Add call_once flag to initialize debugger (llvm#80786)

I tried adding a new unit test to the core test
suite (llvm#79533) but it broke the
test suite on AArch64 Linux due to hitting an assertion for calling
`Debugger::Initialize` more than once. When the unit test suite is
invoked as a standalone binary the test suite state is shared, and
`Debugger::Initialize` gets called in `DiagnosticEventTest.cpp` before
being called in `ProgressReportTest.cpp`.

`DiagnosticEventTest.cpp` uses a call_once flag to initialize the
debugger but it's local to that test. This commit adds a once_flag to
`TestUtilities` so that `Debugger::Initialize` can be called once by the
tests that use it.

lldb/unittests/TestingSupport/TestUtilities.cpp

@@ -19,6 +19,7 @@ using namespace lldb_private;
 
 extern const char *TestMainArgv0;
 
+std::once_flag TestUtilities::g_debugger_initialize_flag;
 std::string lldb_private::GetInputFilePath(const llvm::Twine &name) {
   llvm::SmallString<128> result = llvm::sys::path::parent_path(TestMainArgv0);
   llvm::sys::fs::make_absolute(result);

lldb/unittests/TestingSupport/TestUtilities.h

@@ -31,6 +31,11 @@
 namespace lldb_private {
 std::string GetInputFilePath(const llvm::Twine &name);
 
+class TestUtilities {
+public:
+  static std::once_flag g_debugger_initialize_flag;
+};
+
 class TestFile {
 public:
   static llvm::Expected<TestFile> fromYaml(llvm::StringRef Yaml);
@@ -51,6 +56,6 @@ class TestFile {
 
   std::string Buffer;
 };
-}
+} // namespace lldb_private
 
 #endif