[clang] [analyzer] Check the correct first and last elements in cstring.UninitializedRead (PR #95408)

[Donát Nagy via cfe-commits]

================
@@ -26,16 +50,12 @@ void top(char *dst) {
 
 void *mempcpy(void *restrict s1, const void *restrict s2, size_t n);
 
-void mempcpy14() {
+void mempcpy13() {
   int src[] = {1, 2, 3, 4};
   int dst[5] = {0};
   int *p;
 
-  p = mempcpy(dst, src, 4 * sizeof(int)); // expected-warning{{Bytes string function accesses uninitialized/garbage values}}
-   // FIXME: This behaviour is actually surprising and needs to be fixed, 
-   // mempcpy seems to consider the very last byte of the src buffer uninitialized
-   // and returning undef unfortunately. It should have returned unknown or a conjured value instead.
-
+  p = mempcpy(dst, src, 4 * sizeof(int));
   clang_analyzer_eval(p == &dst[4]); // no-warning (above is fatal)
----------------
NagyDonat wrote:

Delete or update the comment on this line.

https://github.com/llvm/llvm-project/pull/95408