Factor out duplication between lvalue-to-rvalue conversions and variable

assignments in constant expressions. No significant functionality changes
(slight improvement to potential constant expression checking). 


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181170 91177308-0d34-0410-b5e6-96231b3b80d8

test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp

@@ -274,11 +274,13 @@ namespace std_example {
     int a; // expected-error {{must be initialized}}
     return a;
   }
-  // FIXME: Once we support variable mutation, this can produce a
-  // constant expression.
-  constexpr int prev(int x) { // expected-error {{never produces a constant expression}}
-    return --x; // expected-note {{subexpression}}
+  constexpr int prev(int x) {
+    return --x;
   }
+#if 1 // FIXME: !defined CXX1Y
+  // expected-error@-4 {{never produces a constant expression}}
+  // expected-note@-4 {{subexpression}}
+#endif
   constexpr int g(int x, int n) {
     int r = 1;
     while (--n > 0) r *= x;

test/SemaCXX/constant-expression-cxx1y.cpp

@@ -232,6 +232,11 @@ namespace potential_const_expr {
     int z = 0;
     return 100 / (set(z), 0); // expected-note {{division by zero}}
   }
+  int n; // expected-note {{declared here}}
+  constexpr int ref() { // expected-error {{never produces a constant expression}}
+    int &r = n;
+    return r; // expected-note {{read of non-const variable 'n'}}
+  }
 }
 
 namespace subobject {