[lldb][core] Fix getting summary of a variable pointing to r/o memory (llvm#139196)

Motivation example:

```
> lldb -c altmain2.core
...
(lldb) var F
(const char *) F = 0x0804a000 ""
```

The variable `F` points to a read-only memory page not dumped to the
core file, so `Process::ReadMemory()` cannot read the data. The patch
switches to `Target::ReadMemory()`, which can read data both from the
process memory and the application binary.

Author: igorkudrin

lldb/source/ValueObject/ValueObject.cpp

@@ -734,11 +734,13 @@ size_t ValueObject::GetPointeeData(DataExtractor &data, uint32_t item_idx,
     } break;
     case eAddressTypeLoad: {
       ExecutionContext exe_ctx(GetExecutionContextRef());
-      Process *process = exe_ctx.GetProcessPtr();
-      if (process) {
+      if (Target *target = exe_ctx.GetTargetPtr()) {
         heap_buf_ptr->SetByteSize(bytes);
-        size_t bytes_read = process->ReadMemory(
-            addr + offset, heap_buf_ptr->GetBytes(), bytes, error);
+        Address target_addr;
+        target_addr.SetLoadAddress(addr + offset, target);
+        size_t bytes_read =
+            target->ReadMemory(target_addr, heap_buf_ptr->GetBytes(), bytes,
+                               error, /*force_live_memory=*/true);
         if (error.Success() || bytes_read > 0) {
           data.SetData(data_sp);
           return bytes_read;

lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py

@@ -977,6 +977,34 @@ def test_get_core_file_api(self):
         self.assertEqual(process.GetCoreFile().GetFilename(), core_file_name)
         self.dbg.DeleteTarget(target)
 
+    @skipIfLLVMTargetMissing("X86")
+    def test_read_only_cstring(self):
+        """
+        Test that we can show the summary for a cstring variable that points
+        to a read-only memory page which is not dumped to a core file.
+        """
+        target = self.dbg.CreateTarget("altmain2.out")
+        process = target.LoadCore("altmain2.core")
+        self.assertTrue(process, PROCESS_IS_VALID)
+
+        frame = process.GetSelectedThread().GetFrameAtIndex(0)
+        self.assertEqual(frame.GetFunctionName(), "_start")
+
+        var = frame.FindVariable("F")
+
+        # The variable points to a read-only segment that is not dumped to
+        # the core file and thus 'process.ReadCStringFromMemory()' cannot get
+        # the value.
+        error = lldb.SBError()
+        cstr = process.ReadCStringFromMemory(var.GetValueAsUnsigned(), 256, error)
+        self.assertFailure(error, error_str="core file does not contain 0x804a000")
+        self.assertEqual(cstr, "")
+
+        # Nevertheless, when getting the summary, the value can be read from the
+        # application binary.
+        cstr = var.GetSummary()
+        self.assertEqual(cstr, '"_start"')
+
     def check_memory_regions(self, process, region_count):
         region_list = process.GetMemoryRegions()
         self.assertEqual(region_list.GetSize(), region_count)