Reland [LLDB] Fix MS STL variant with non-trivial types and PDB (#176189)

Relands #171489 which was reverted due to a test failure on GreenDragon.

As suggested in
https://github.com/llvm/llvm-project/pull/171489#issuecomment-3754098800,
I'm checking for `std::string` on Darwin and `std::basic_string<char>`
otherwise, however I can't test that locally.

lldb/source/Plugins/Language/CPlusPlus/MsvcStlVariant.cpp

diff --git a/lldb/source/Plugins/Language/CPlusPlus/MsvcStlVariant.cpp b/lldb/source/Plugins/Language/CPlusPlus/MsvcStlVariant.cpp
index 3e7647b..55e9642 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/MsvcStlVariant.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/MsvcStlVariant.cpp
@@ -67,12 +67,18 @@
 ValueObjectSP GetNthStorage(ValueObject &outer, int64_t index) {
   // We need to find the std::_Variant_storage base class.
 
-  // -> std::_SMF_control (typedef to std::_Variant_base)
-  ValueObjectSP container_sp = outer.GetSP()->GetChildAtIndex(0);
-  if (!container_sp)
+  // Navigate "down" to std::_Variant_base by finding the holder of "_Which".
+  // This might be down a few levels if a variant member isn't trivially
+  // destructible/copyable/etc.
+  ValueObjectSP which_sp = outer.GetChildMemberWithName("_Which");
+  if (!which_sp)
     return nullptr;
-  // -> std::_Variant_storage
-  container_sp = container_sp->GetChildAtIndex(0);
+  ValueObject *parent = which_sp->GetParent();
+  if (!parent)
+    return nullptr;
+
+  // Now go to std::_Variant_storage.
+  ValueObjectSP container_sp = parent->GetChildAtIndex(0);
   if (!container_sp)
     return nullptr;
 

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/TestDataFormatterStdVariant.py

diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/TestDataFormatterStdVariant.py b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/TestDataFormatterStdVariant.py
index 1ae07a9..cd03c12 100644
--- a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/TestDataFormatterStdVariant.py
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/TestDataFormatterStdVariant.py
@@ -50,6 +50,14 @@
             ],
         )
 
+        self.expect_expr(
+            "v4",
+            result_summary=" Active Type = int ",
+            result_children=[
+                ValueCheck(name="Value", value="4"),
+            ],
+        )
+
         lldbutil.continue_to_breakpoint(self.process, bkpt)
 
         self.expect(
@@ -69,6 +77,23 @@
             substrs=["v3 =  Active Type = char  {", "Value = 'A'", "}"],
         )
 
+        if self.getDebugInfo() == "pdb":
+            string_name = (
+                "std::basic_string<char, std::char_traits<char>, std::allocator<char>>"
+            )
+        elif self.platformIsDarwin():
+            string_name = "std::string"
+        else:
+            string_name = "std::basic_string<char>"
+
+        self.expect_expr(
+            "v4",
+            result_summary=f" Active Type = {string_name} ",
+            result_children=[
+                ValueCheck(name="Value", summary='"a string"'),
+            ],
+        )
+
         self.expect("frame variable v_valueless", substrs=["v_valueless =  No Value"])
 
         self.expect(

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/main.cpp

diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/main.cpp b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/main.cpp
index 620b97b..9983104 100644
--- a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/main.cpp
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/main.cpp
@@ -49,6 +49,8 @@
       S>
       v_300_types_valueless;
 
+  std::variant<int, bool, std::string> v4 = 4;
+
   v_valueless = 5;
   v_300_types_valueless.emplace<0>(10);
 
@@ -70,6 +72,9 @@
   // state when we change its value.
   v1 = 2.0;
   d = std::get<double>(v1);
+
+  v4 = "a string";
+
   printf("%f\n", d); // break here
 
   try {