[YAMLParser] Add multi-line literal folding support

Last year I was working at Swift to add support for [Localization of Compiler Diagnostic Messages](https://forums.swift.org/t/localization-of-compiler-diagnostic-messages/36412/41). We are currently using YAML as the new diagnostic format. The LLVM::YAMLParser didn't have a support for multiline string literal folding and it's crucial to have that for the diagnostic message to help us keep up with the 80 columns rule. Therefore, I decided to add a multiline string literal folding support to the YAML parser.

Patch By: @HassanElDesouky (Hassan ElDesouky)

Differential Revision: https://reviews.llvm.org/D102590