[Parser] Decouple the parser from AST creation (part 1)

[jansvoboda11]

Instead of creating the AST directly in the parser (and libSyntax or SwiftSyntax via SyntaxParsingContext), make Parser to explicitly create a tree of ParsedSyntaxNodes. Their OpaqueSyntaxNodes can be either libSyntax or SwiftSyntax. If AST is needed, it can be generated from the libSyntax tree.
This PR contains the infrastructure to achieve the goal in an incremental way and also refactored literal parsing.

This PR is a part of a Google Summer of Code 2019 project.

[rintaro]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 44d7769

[akyrtzi]

@swift-ci Please test

[beccadax]

@jansvoboda11 Unfortunately, I had to revert this change because a few of our CI bots had trouble building after it was merged.

From what I can see, the problem is that Parser.h now includes ParsedSyntaxNodes.h; this is a generated header and it looks like it doesn't always get generated before Parser.h is needed. You can test this locally by removing the build/[configuration]/swift-macosx-x86_64/ directory between build-script runs. To fix this, you might be able to avoid including ParsedSyntaxNodes.h in Parser.h; if you can't, you'll need to tweak the CMake configuration to generate ParsedSyntaxNodes.h earlier. Once you've finished this work, @akyrtzi or another committer can use "clean smoke test" instead of just "smoke test" to check that it worked.

I also noticed a leaks test run with failures that may stem from this PR. I don't think there's a way to ask the CI bot to run a leaks test, but you can use --enable-lsan with the usual options like -t to run one locally.

[jansvoboda11]

Thanks for the heads-up @brentdax, I'll look into it ASAP.

[davezarzycki]

In my experience with unified builds, the project CMake files are already brittle in this regard. I think the following needs to be added in a few more CMakeLists.txt files but I haven't looked into it thoroughly:

# Add generated libSyntax headers to global dependencies.
list(APPEND LLVM_COMMON_DEPENDS swift-syntax-generated-headers)
list(APPEND LLVM_COMMON_DEPENDS swift-parse-syntax-generated-headers)