Workaround for EvalInfo ctor for MSVC 2017

[yaxunl]

Summary

Current EvalInfo ctor causes EnableNewConstInterp to be true even though
it is supposed to be false on MSVC 2017. This is because a virtual function
getLangOpts() is called in member initializer lists, whereas on MSVC
member ctors are called before function virtual function pointers are
initialized.

https://docs.microsoft.com/en-us/cpp/cpp/constructors-cpp?view=vs-2019#order_of_construction

This patch fixes that.

[yaxunl]

add a test

[hliao]

LGTM, anyway calling a virtual function inside the constructor is not encouraged.

clang/lib/AST/ExprConstant.cpp

public:
     // in such constructs, not just overflow.
     bool checkingForUndefinedBehavior() const override {
       return CheckingForUndefinedBehavior;
     }
 
     EvalInfo(const ASTContext &C, Expr::EvalStatus &S, EvaluationMode Mode)
         : Ctx(const_cast<ASTContext &>(C)), EvalStatus(S), CurrentCall(nullptr),
           CallStackDepth(0), NextCallIndex(1),
-          StepsLeft(getLangOpts().ConstexprStepLimit),
-          ForceNewConstInterp(getLangOpts().ForceNewConstInterp),
+          StepsLeft(C.getLangOpts().ConstexprStepLimit),
+          ForceNewConstInterp(C.getLangOpts().ForceNewConstInterp),
           EnableNewConstInterp(ForceNewConstInterp ||
-                               getLangOpts().EnableNewConstInterp),
+                               C.getLangOpts().EnableNewConstInterp),
           BottomFrame(*this, SourceLocation(), nullptr, nullptr, nullptr),
           EvaluatingDecl((const ValueDecl *)nullptr),
           EvaluatingDeclValue(nullptr), HasActiveDiagnostic(false),
           HasFoldFailureDiagnostic(false), InConstantContext(false),
           EvalMode(Mode) {}
 
     ~EvalInfo() {
       discardCleanups();

clang/test/Sema/eval-info.c

+// RUN: %clang_cc1 %s -fsyntax-only -triple x86_64-unknown-windows-msvc -verify
+
+// expected-no-diagnostics
+
+// Make sure the new constant interpolator is not enabled unintentionally
+// to cause assertion.
+typedef enum x {
+  a = 1,
+} x;