[clang-repl] Add basic multiline input support

[sunho]

Summary

Add basic multi-line input support to clang-repl by using newly available SourceFileGrower interface of Preprocessor.

Compared to cling, this change uses fully fledged clang preprocessor (not MetaLexer) so it supports very complicated macro expressions.

NOTE: this is the related to https://discourse.llvm.org/t/rfc-flexible-lexer-buffering-for-handling-incomplete-input-in-interactive-c-c/64180

[sunho]

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[sunho]

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[sunho]

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[sunho]

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[sunho]

Update

[v.g.vassilev]

clang/lib/Interpreter/IncrementalParser.cpp

260

I think we can simplify the heuristics here by asking if the end source location of the last created declaration is the same as the parser/lexer current token that is observed before EOF. If that's the case, then we are done. If not we can request more input.