[Support] Do not ignore unterminated open { in formatv

- 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 behvaior indentical between assert and
  release builds and also help catch the error more convienent (as
several uses of this function are in TableGen and its if often built in
release mode even in debug builds, using

Author: jurahul

llvm/lib/Support/FormatVariadic.cpp

@@ -112,10 +112,8 @@ formatv_object_base::splitLiteralAndReplacement(StringRef Fmt) {
     // undefined and that we consider it an error.
     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());
+      return std::make_pair(ReplacementItem{"Unterminated open {"},
+                            StringRef());
     }
 
     // Even if there is a closing brace, if there is another open brace before