[MSVC Compat] Don't evaluate member base expressions w/o side effects

A member expression's base doesn't always have an impact on what the
member decl would evaluate to.  In such a case, the base is used as a
poor man's scope qualifier.

This fixes PR26738.

Differential Revision: http://reviews.llvm.org/D17619

llvm-svn: 261975
diff --git a/clang/test/SemaCXX/ms-const-member-expr.cpp b/clang/test/SemaCXX/ms-const-member-expr.cpp
new file mode 100644
index 0000000..72cfe76
--- /dev/null
+++ b/clang/test/SemaCXX/ms-const-member-expr.cpp
@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 %s -std=c++11 -fms-compatibility -fsyntax-only -verify
+
+struct S {
+  enum { E = 1 };
+  static const int sdm = 1;
+};
+
+void f(S *s) {
+  char array[s->E] = { 0 };
+}
+
+extern S *s;
+constexpr int e1 = s->E;
+
+S *side_effect();  // expected-note{{declared here}}
+constexpr int e2 = // expected-error{{must be initialized by a constant expression}}
+    side_effect()->E; // expected-note{{cannot be used in a constant expression}}
+
+constexpr int e4 = s->sdm;