[clang][Interp] Always decay root array pointers to the first element

This is similar to what the current interpreter does.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -2194,7 +2194,7 @@ inline bool ArrayDecay(InterpState &S, CodePtr OpPC) {
   if (!CheckRange(S, OpPC, Ptr, CSK_ArrayToPointer))
     return false;
 
-  if (!Ptr.isUnknownSizeArray() || Ptr.isDummy()) {
+  if (Ptr.isRoot() || !Ptr.isUnknownSizeArray() || Ptr.isDummy()) {
     S.Stk.push<Pointer>(Ptr.atIndex(0));
     return true;
   }

clang/test/AST/Interp/arrays.cpp

@@ -615,3 +615,11 @@ namespace OnePastEndSub {
   constexpr A a[3][3];
   constexpr int diff2 = &a[1][3] - &a[1][0]; /// Used to crash.
 }
+
+static int same_entity_2[3];
+constexpr int *get2() {
+  // This is a redeclaration of the same entity, even though it doesn't
+  // inherit the type of the prior declaration.
+  extern int same_entity_2[];
+  return same_entity_2;
+}

clang/test/AST/Interp/literals.cpp

@@ -1209,7 +1209,7 @@ namespace incdecbool {
 constexpr int externvar1() { // both-error {{never produces a constant expression}}
   extern char arr[]; // ref-note {{declared here}}
    return arr[0]; // ref-note {{read of non-constexpr variable 'arr'}} \
-                  // expected-note {{array-to-pointer decay of array member without known bound is not supported}}
+                  // expected-note {{indexing of array without known bound}}
 }
 #endif