[-Wunsafe-buffer-usage][NFC] Note future work for isSafeArraySubscript

-Warray-bounds implemented in Sema::CheckArrayAccess() already solves a similar
(opposite) problem and is battle-tested.

Adding -Wunsafe-buffer-usage diagnostics to Sema is a non starter as we need to emit
both the warnings and fixits and the performance impact of the fixit machine is
unacceptable for Sema.

CheckArrayAccess() as is doesn't distinguish between "safe" and "unknown" array
accesses. It also mixes the analysis that decides if an index is out of bounds
with crafting the diagnostics.

A refactor of CheckArrayAccess() might serve both the original purpose
and help us avoid false-positive with -Wunsafe-buffer-usage on constant
size arrrays.

Author: jkorous-apple

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -408,6 +408,12 @@ AST_MATCHER(CXXConstructExpr, isSafeSpanTwoParamConstruct) {
 }
 
 AST_MATCHER(ArraySubscriptExpr, isSafeArraySubscript) {
+  // FIXME: Proper solution:
+  //  - refactor Sema::CheckArrayAccess
+  //    - split safe/OOB/unknown decision logic from diagnostics emitting code
+  //    -  e. g. "Try harder to find a NamedDecl to point at in the note." already duplicated
+  //  - call both from Sema and from here
+
   const DeclRefExpr * BaseDRE = dyn_cast_or_null<DeclRefExpr>(Node.getBase()->IgnoreParenImpCasts());
   if (!BaseDRE)
     return false;