[Support] Do not ignore unterminated open { in formatv (#104688)

- When an unterminated open { is detected in the format string, instead
of asserting and ignoring the error, replace that string with another to
indicate the error, and remove the assert as well.
- This will make the error evident in both assert and release builds and
make observing the error more convenient (as several uses of this
function are in TableGen and it is often built in release mode even in
debug builds)

Author: jurahul

llvm/lib/Support/FormatVariadic.cpp

@@ -107,15 +107,18 @@ formatv_object_base::splitLiteralAndReplacement(StringRef Fmt) {
       StringRef Right = Fmt.drop_front(NumEscapedBraces * 2);
       return std::make_pair(ReplacementItem{Middle}, Right);
     }
-    // An unterminated open brace is undefined.  We treat the rest of the string
-    // as a literal replacement, but we assert to indicate that this is
-    // undefined and that we consider it an error.
+    // An unterminated open brace is undefined. Assert to indicate that this is
+    // undefined and that we consider it an error. When asserts are disabled,
+    // build a replacement item with an error message.
     std::size_t BC = Fmt.find_first_of('}');
     if (BC == StringRef::npos) {
       assert(
           false &&
-          "Unterminated brace sequence.  Escape with {{ for a literal brace.");
-      return std::make_pair(ReplacementItem{Fmt}, StringRef());
+          "Unterminated brace sequence. Escape with {{ for a literal brace.");
+      return std::make_pair(
+          ReplacementItem{"Unterminated brace sequence. Escape with {{ for a "
+                          "literal brace."},
+          StringRef());
     }
 
     // Even if there is a closing brace, if there is another open brace before