Update default.xml

Update pinned LLVM repository.

Author: compnerd

default.xml

@@ -4,7 +4,7 @@
 
   <default revision="main" sync-c="true" sync-tags="false" />
 
-  <project remote="github" name="apple/llvm-project" path="llvm-project" revision="stable/20211026" />
+  <project remote="github" name="apple/llvm-project" path="llvm-project" revision="stable/20220421" />
   <project remote="github" name="apple/indexstore-db" path="indexstore-db" />
   <project remote="github" name="apple/sourcekit-lsp" path="sourcekit-lsp" />
   <project remote="github" name="apple/swift" path="swift" />