Add a FIXME comment; NFC

The constant expression engines use a list of note diagnostics to
report back whether an evaluation is a valid constant expression or
not. This requires the engines to generate those note diagnostics, and
that can be expensive in cases where we're converting an APValue to a
string as a streamed argument to the note. If we're going to display
the note, then that expense is warranted. However, there are operations
looking for a yes/no answer to whether something is a constant
expression and won't display the diagnostics at all. Those uses are
paying an expense they shouldn't have to.

Author: AaronBallman

clang/include/clang/AST/Expr.h

@@ -607,6 +607,13 @@ class Expr : public ValueStmt {
     /// foldable. If the expression is foldable, but not a constant expression,
     /// the notes will describes why it isn't a constant expression. If the
     /// expression *is* a constant expression, no notes will be produced.
+    ///
+    /// FIXME: this causes significant performance concerns and should be
+    /// refactored at some point. Not all evaluations of the constant
+    /// expression interpreter will display the given diagnostics, this means
+    /// those kinds of uses are paying the expense of generating a diagnostic
+    /// (which may include expensive operations like converting APValue objects
+    /// to a string representation).
     SmallVectorImpl<PartialDiagnosticAt> *Diag = nullptr;
 
     EvalStatus() = default;