[clang][Interp] Fix Pointer::toAPValue() for expressions

[tbaeder]

Summary

This is now relevant because we generate pointers for expressions more often, since MaterializeTemporaryExprs work.

[tbaeder]

Ping

[tbaeder]

Ping

[aaron.ballman]

Test coverage?

[tbaeder]

It's a bit involved since without working MaterializeTemporaryExprs, this code is a no-op, but without this code, the MTEs don't properly work, usually.

[aaron.ballman]

LG given that other test coverage exists for this.

clang/lib/AST/Interp/Pointer.cpp

else
       llvm_unreachable("Invalid allocation type");
 
     // Not a null pointer.
     IsNullPtr = false;
 
     if (isUnknownSizeArray()) {
       IsOnePastEnd = false;
       Offset = CharUnits::Zero();
+    } else if (getFieldDesc()->asExpr()) {
+      // Pointer pointing to a an expression.
+      IsOnePastEnd = false;
+      Offset = CharUnits::Zero();
     } else {
       // TODO: compute the offset into the object.
       Offset = CharUnits::Zero();
 
       // Build the path into the object.
       Pointer Ptr = *this;
       while (Ptr.isField() || Ptr.isArrayElement()) {
         if (Ptr.isArrayElement()) {