I have published the patches that implement this RFC:
⚙ D143142 [clang][lex] Enable Lexer to grow its buffer (the main one)
⚙ D143144 [clang][lex] Add TryGrowLexerBuffer/SourceFileGrower
⚙ D143148 [clang-repl] Add basic multiline input support
Here’s also the link to my talk that provides more detailed information about the context: Lazy Lexer Presentation - YouTube
Please take a look.
Best,