[lldb] Test 'v' support for direct ivar access (NFC)

[kastiglione]

Summary

Add basic tests for frame variable's ability to direct access fields of this and
ivars of self.

[kastiglione]

Split into separate cpp/this and objc/self tests

lldb/test/API/commands/frame/var/direct-ivar/Makefile

+OBJCXX_SOURCES := main.mm
+include Makefile.rules

lldb/test/API/commands/frame/var/direct-ivar/TestFrameVarDirectIvar.py

+import lldb
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class TestCase(TestBase):
+    def test(self):
+        self.build()
+        _, process, _, _ = lldbutil.run_to_source_breakpoint(
+            self, "break here", lldb.SBFileSpec("main.mm")
+        )
+
+        self.expect("frame variable m_field", startstr="(int) m_field = 30")
+        process.Continue()
+        self.expect("frame variable _ivar", startstr="(int) _ivar = 41")

lldb/test/API/commands/frame/var/direct-ivar/main.mm

+#include <objc/NSObject.h>
+
+struct Structure {
+  int m_field;
+  void fun() {
+    // break here
+  }
+};
+
+@interface Classic : NSObject {
+@public
+  int _ivar;
+}
+@end
+
+@implementation Classic
+- (void)fun {
+  // break here
+}
+@end
+
+int main() {
+  Structure s;
+  s.m_field = 30;
+  s.fun();
+
+  Classic *c = [Classic new];
+  c->_ivar = 41;
+  [c fun];
+}