[clang-diff] Add initial implementation
This is the first commit for the "Clang-based C/C++ diff tool" GSoC project.
ASTDiff is a new library that computes a structural AST diff between two ASTs
using the gumtree algorithm. Clang-diff is a new Clang tool that will show
the structural code changes between different ASTs.
Patch by Johannes Altmanninger!
Differential Revision: https://reviews.llvm.org/D34329
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308731
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