libcxx: Speed up import script
```
thegeorg@thegeorg-maps:~/arcadia/contrib/libs/cxxsupp/libcxx$ time ./import
./import  0.88s user 0.83s system 2% cpu 1:08.48 total
```
```
thegeorg@thegeorg-maps:~/arcadia-git/contrib/libs/cxxsupp/libcxx$ time ./import
./import  0.16s user 0.17s system 2% cpu 12.268 total
```
#!/bin/sh -e
rev=338120
+output_dir="libcxx-r$rev"
-svn export https://llvm.org/svn/llvm-project/libcxx/trunk@$rev libcxx-r$rev
-cd libcxx-r$rev
+svn checkout --depth=files --non-interactive https://llvm.org/svn/llvm-project/libcxx/trunk@$rev "$output_dir"
+cd "$output_dir"
-rm -r benchmarks cmake docs fuzzing lib test utils www CMakeLists.txt TODO.TXT .arcconfig .gitignore include/CMakeLists.txt appveyor.yml appveyor-reqs-install.cmd
+svn update --revision=$rev include src
+svn update --set-depth=exclude --revision=$rev CMakeLists.txt TODO.TXT .arcconfig .gitignore include/CMakeLists.txt appveyor.yml appveyor-reqs-install.cmd
+rm -rf .svn
