[ExpressionParser] Fix evaluation failures due to mismatch in C++ versions.

Clang recently switched to C++14 (with GNU extensions) as the default
dialect, but LLDB didn't catch up. This causes failures as LLDB still
evaluates ObjectiveC expressions as Objective C++ using C++98 as standard.

There are things not available in C++98, including, e.g. nullptr.
In some cases Objective-C `nil` is defined as `nullptr` so this causes
an evaluation failure. Switch the default to overcome this issue
(actually, currently lldb evaluates both C++11 and C++14 as C++11,
but that seems a larger change and definitely could be re-evaluated
in the future).

No test as this is currently failing on the LLDB bots after the clang
switch (so, de facto, there's a test already for it).

<rdar://problem/36011995>

llvm-svn: 320761

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

@@ -381,7 +381,15 @@ ClangExpressionParser::ClangExpressionParser(ExecutionContextScope *exe_scope,
     // For now, the expression parser must use C++ anytime the
     // language is a C family language, because the expression parser
     // uses features of C++ to capture values.
+
+    // Clang now sets as default C++14 as the default standard (with
+    // GNU extensions), so we do the same here to avoid mismatches that
+    // cause compiler error when evaluating expressions (e.g. nullptr
+    // not found as it's a C++11 feature). Currently lldb evaluates
+    // C++14 as C++11 (see two lines below) so we decide to be consistent
+    // with that, but this could be re-evaluated in the future.
     m_compiler->getLangOpts().CPlusPlus = true;
+    m_compiler->getLangOpts().CPlusPlus11 = true;
     break;
   case lldb::eLanguageTypeObjC:
     m_compiler->getLangOpts().ObjC1 = true;