Rollup merge of rust-lang#117311 - RalfJung:unpretty-thir-help, r=petrochenkov

-Zunpretty help: add missing possible values

`-Zunpretty` accepts "thir-tree" and "thir-flat", but that was not shown in `-Zhelp`.

Author: workingjubilee

compiler/rustc_session/src/options.rs

@@ -1899,6 +1899,7 @@ written to standard error output)"),
         `hir` (the HIR), `hir,identified`,
         `hir,typed` (HIR with types for each node),
         `hir-tree` (dump the raw HIR),
+        `thir-tree`, `thir-flat`,
         `mir` (the MIR), or `mir-cfg` (graphviz formatted MIR)"),
     unsound_mir_opts: bool = (false, parse_bool, [TRACKED],
         "enable unsound and buggy MIR optimizations (default: no)"),