[lldb][test] Fix libstdc++ std::variant formatter for empty variant

Instead of using the byte-size to make a guess at what the
`std::variant_npos` value is, just look it up in debug-info.

Unblocks llvm#147253

Author: Michael137

lldb/examples/synthetic/gnu_libstdcpp.py

@@ -889,17 +889,12 @@ def VariantSummaryProvider(valobj, dict):
     if not (index_obj and index_obj.IsValid() and data_obj and data_obj.IsValid()):
         return "<Can't find _M_index or _M_u>"
 
-    def get_variant_npos_value(index_byte_size):
-        if index_byte_size == 1:
-            return 0xFF
-        elif index_byte_size == 2:
-            return 0xFFFF
-        else:
-            return 0xFFFFFFFF
+    npos = valobj.GetTarget().FindFirstGlobalVariable("std::variant_npos")
+    if not npos:
+        return "<Can't find std::variant_npos sentinel>"
 
-    npos_value = get_variant_npos_value(index_obj.GetByteSize())
     index = index_obj.GetValueAsUnsigned(0)
-    if index == npos_value:
+    if index == npos.GetValueAsUnsigned(0):
         return " No Value"
 
     # Strip references and typedefs.

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py

@@ -2,7 +2,6 @@
 Test lldb data formatter for LibStdC++ std::variant.
 """
 
-
 import lldb
 from lldbsuite.test.decorators import *
 from lldbsuite.test.lldbtest import *
@@ -62,17 +61,13 @@ def test_with_run_command(self):
             "frame variable v3",
             substrs=["v3 =  Active Type = char  {", "Value = 'A'", "}"],
         )
-        """
-        TODO: temporarily disable No Value tests as they seem to fail on ubuntu/debian
-        bots. Pending reproduce and investigation.
 
         self.expect("frame variable v_no_value", substrs=["v_no_value =  No Value"])
 
         self.expect(
             "frame variable v_many_types_no_value",
             substrs=["v_many_types_no_value =  No Value"],
         )
-        """
 
     @add_test_categories(["libstdcxx"])
     def test_invalid_variant_index(self):

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

@@ -49,6 +49,7 @@ int main() {
   v1 = 12; // v contains int
   v1_typedef = v1;
   v_v1 = v1;
+  v1_typedef = v1;
   int i = std::get<int>(v1);
   printf("%d\n", i); // break here