[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_cpp_this(self):
+        self.build()
+        lldbutil.run_to_source_breakpoint(self, "this", lldb.SBFileSpec("main.mm"))
+        self.expect("frame variable m_field", startstr="(int) m_field = 30")
+
+    def test_objc_self(self):
+        self.build()
+        lldbutil.run_to_source_breakpoint(self, "self", lldb.SBFileSpec("main.mm"))
+        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;
+  int fun() { return this->m_field; }
+};
+
+@interface Classic : NSObject {
+@public
+  int _ivar;
+}
+@end
+
+@implementation Classic
+- (int)fun {
+  return self->_ivar;
+}
+@end
+
+int main() {
+  Structure s;
+  s.m_field = 30;
+  s.fun();
+
+  Classic *c = [Classic new];
+  c->_ivar = 41;
+  [c fun];
+}