Rollup merge of rust-lang#153229 - BennoLossin:frt-fmt, r=ytmimi

rustfmt: add test for field representing type builtin syntax

As requested by @ytmimi in rust-lang#152730 (comment).

Not sure if you want/need a more complex example. Just let me know.

Author: matthiaskrgr

src/tools/rustfmt/tests/target/field-representing-types.rs

@@ -0,0 +1,2 @@
+type FRT = builtin # field_of(Type, ident);
+type FRT2 = builtin # field_of(Type<A, B>, ident);