Modified format logic so that braces are not allowed in custom format specifiers...

Modified format logic so that braces are not allowed in custom format specifiers (dotnet/coreclr#23062)

* Modified format logic so that braces are not allowed in custom format specifiers.

* Disabled failing test temporarily and fixed code style issue.

PR needed to pass test: https://github.com/dotnet/corefx/pull/35820

* Slight changes to logic to better align the diff.

* Fixed counting error.

* Fix nullable

* Move exclusion near related test

* Add skip to new issue file

* Delete CoreFX.issues.json

Commit migrated from https://github.com/dotnet/coreclr/commit/5d8fea006d98d98ba269056de519199f354bcf1a