Add /build-rust-analyzer/ to .gitignore

To avoid rust-analyzer and rustc having to wait for each other,
the dev guide mentions using another build directory for RA.

We should also put this into the .gitignore, just like the normal `build`.

Author: Noratrieb

.gitignore

@@ -42,6 +42,7 @@ no_llvm_build
 /llvm/
 /mingw-build/
 /build/
+/build-rust-analyzer/
 /dist/
 /unicode-downloads
 /target