[DWARFASTParserClang] Start with member offset of 0 for members of union types.

[sivachandra]

Summary

GCC does not emit DW_AT_data_member_location for members of a union.
Starting with a 0 value for member locations helps is reading union types
in such cases.

[clayborg]

Looks good.

packages/Python/lldbsuite/test/lang/c/unions/Makefile

+LEVEL = ../../../make
+
+C_SOURCES := main.c
+
+include $(LEVEL)/Makefile.rules

packages/Python/lldbsuite/test/lang/c/unions/TestUnionMembers.py

+import lldb
+from lldbsuite.test.lldbtest import *
+import lldbsuite.test.lldbutil as lldbutil
+
+class TestUnionMembers(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    def test_union_members(self):
+        self._load_exe()
+
+        # Set breakpoints
+        bp = self.target.BreakpointCreateBySourceRegex("Break here", self.src_file_spec)
+        self.assertTrue(bp.IsValid() and bp.GetNumLocations() >= 1, VALID_BREAKPOINT)
+
+        # Launch the process
+        self.process = self.target.LaunchSimple(None, None, self.get_process_working_directory())
+        self.assertTrue(self.process.IsValid(), PROCESS_IS_VALID)
+        self.assertTrue(self.process.GetState() == lldb.eStateStopped, PROCESS_STOPPED)
+
+        thread = lldbutil.get_stopped_thread(self.process, lldb.eStopReasonBreakpoint)
+        self.assertTrue(thread.IsValid())
+        frame = thread.GetSelectedFrame()
+        self.assertTrue(frame.IsValid())
+
+        val = frame.EvaluateExpression("u");
+        self.assertTrue(val.IsValid())
+        val = frame.EvaluateExpression("u.s");
+        self.assertTrue(val.IsValid())
+        self.assertEqual(val.GetNumChildren(), 2)
+
+    def _load_exe(self):
+        self.build()
+
+        cwd = os.getcwd()
+
+        src_file = os.path.join(cwd, "main.c")
+        self.src_file_spec = lldb.SBFileSpec(src_file)
+        self.assertTrue(self.src_file_spec.IsValid(), "breakpoint file")
+
+        # Get the path of the executable
+        exe_path  = os.path.join(cwd, 'a.out')
+
+        # Load the executable
+        self.target = self.dbg.CreateTarget(exe_path)
+        self.assertTrue(self.target.IsValid(), VALID_TARGET)

packages/Python/lldbsuite/test/lang/c/unions/main.c

+#include <stdint.h>
+
+union S
+{
+    int32_t n;     // occupies 4 bytes
+    uint16_t s[2]; // occupies 4 bytes
+    uint8_t c;     // occupies 1 byte
+};                 // the whole union occupies 4 bytes
+
+int main()
+{
+  union S u;
+
+  u.s[0] = 1234;
+  u.s[1] = 4321;
+
+  return 0; // Break here
+}

source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp

for (DWARFDIE die = parent_die.GetFirstChild(); die.IsValid(); die = die.GetSibling())
                     const char *prop_getter_name = NULL;
                     const char *prop_setter_name = NULL;
                     uint32_t prop_attributes = 0;
 
 
                     bool is_artificial = false;
                     DWARFFormValue encoding_form;
                     AccessType accessibility = eAccessNone;
-                    uint32_t member_byte_offset = UINT32_MAX;
+                    uint32_t member_byte_offset = (parent_die.Tag() == DW_TAG_union_type) ? 0 : UINT32_MAX;
                     size_t byte_size = 0;
                     size_t bit_offset = 0;
                     size_t bit_size = 0;
                     bool is_external = false; // On DW_TAG_members, this means the member is static
                     uint32_t i;
                     for (i=0; i<num_attributes && !is_artificial; ++i)
                     {
                         const dw_attr_t attr = attributes.AttributeAtIndex(i);