[clang] [C++26][clang] Implement P2795R5 'Erroneous behaviour for uninitialized reads' (PR #177614)

[via cfe-commits]

================
@@ -462,6 +477,7 @@ class APValue {
 
   bool isAbsent() const { return Kind == None; }
   bool isIndeterminate() const { return Kind == Indeterminate; }
+  bool isErroneous() const { return Kind == Erroneous; }
----------------
yronglin wrote:

I have added a new `isUninit()` function, WDYT?

https://github.com/llvm/llvm-project/pull/177614