Add some docs on debugging

[varungandhi-src]

For example, if you have a compile_commands.json which is generating an empty LSIF dump, here is a useful baseline to check first and start delta debugging.

---

Configuration: clang-11 and lsif-clang from the sourcegraph/lsif-clang Docker image (e78b83931bd2).

Testing with a basic cpp file.

// tmp.cpp
struct C {
    static int doStuff() { return 1; }
};
int main() {
    return C::doStuff();
}

I generated the compilation database as follows: (inside the Docker container with a shell)

clang++-11 -MJ tmp.cc.json tmp.cpp -c -o tmp.o

This generates a single compilation command database entry, which needs to be massaged into the right format.

sed -e ‘1s/^/[\n/’ -e ‘$s/,$/\n]/’ *.cc.json > compile_commands.json 

Pass this to lsif-clang

lsif-clang compile_commands.json

This generates a dump.lsif file with 53 lines, including lines such as

{“id”:17,“type”:“vertex”,“label”:“hoverResult”,“result”:{“contents”:[{“language”:“cpp”,“value”:“int C::doStuff()“}]}}

---

[varungandhi-src]

Another thing that would be helpful: Add a helper script for bisecting a crash from a compile_commands.json. This came up in a customer conversation where the customer mentioned that you couldn't run lsif-clang on a single file.

[varungandhi-src]

^ Script above is added in #75; the PR also includes usage examples.