Add `llvm` to git ignore (#457)

diff --git a/.gitignore b/.gitignore
index 73fbd2e..be0372c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,6 +18,7 @@
 /gnu
 /upstream
 /fastcomp
+/llvm
 /releases
 /clang
 /emscripten