[clang][Interp] Fix "Initializing" zero-size arrays

getIndex() returns 0 here, so we were trying to initalize the 0th
element.

Fixes #88018

Author: tbaederr

clang/lib/AST/Interp/Pointer.cpp

@@ -177,6 +177,10 @@ void Pointer::initialize() const {
     if (isStatic() && Base == 0)
       return;
 
+    // Nothing to do for these.
+    if (Desc->getNumElems() == 0)
+      return;
+
     InitMapPtr &IM = getInitMap();
     if (!IM)
       IM =

clang/test/AST/Interp/arrays.cpp

@@ -566,3 +566,7 @@ char melchizedek[2200000000];
 typedef decltype(melchizedek[1] - melchizedek[0]) ptrdiff_t;
 constexpr ptrdiff_t d1 = &melchizedek[0x7fffffff] - &melchizedek[0]; // ok
 constexpr ptrdiff_t d3 = &melchizedek[0] - &melchizedek[0x80000000u]; // ok
+
+/// GH#88018
+const int SZA[] = {};
+void testZeroSizedArrayAccess() { unsigned c = SZA[4]; }