LLVM should be included using quotes. (#2952)

This also results in correct dependency discovery (the missing LLVM dep on expression_category).

explorer/ast/BUILD

@@ -157,4 +157,5 @@ cc_library(
 cc_library(
     name = "expression_category",
     hdrs = ["expression_category.h"],
+    deps = ["@llvm-project//llvm:Support"],
 )

explorer/ast/expression_category.h

@@ -5,7 +5,7 @@
 #ifndef CARBON_EXPLORER_AST_EXPRESSION_CATEGORY_H_
 #define CARBON_EXPLORER_AST_EXPRESSION_CATEGORY_H_
 
-#include <llvm/ADT/StringRef.h>
+#include "llvm/ADT/StringRef.h"
 
 namespace Carbon {