cmd/compile: fix reflect naming of local generic types

To disambiguate local types, we append a "·N" suffix to their name and
then trim it off again when producing their runtime type descriptors.

However, if a local type is generic, then we were further appending
the type arguments after this suffix, and the code in types/fmt.go
responsible for trimming didn't know to handle this.

We could extend the types/fmt.go code to look for the "·N" suffix
elsewhere in the type name, but this is risky because it could
legitimately (albeit unlikely) appear in struct field tags.

Instead, the most robust solution is to just change the mangling logic
to keep the "·N" suffix at the end, where types/fmt.go can easily and
reliably trim it.

Note: the "·N" suffix is still visible within the type arguments
list (e.g., the "·3" suffixes in nested.out), because we currently use
the link strings in the type arguments list.

Fixes #54456.

Change-Id: Ie9beaf7e5330982f539bff57b8d48868a3674a37
Reviewed-on: https://go-review.googlesource.com/c/go/+/424901
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/noder/reader.go

@@ -777,8 +777,13 @@ func (dict *readerDict) mangle(sym *types.Sym) *types.Sym {
 		return sym
 	}
 
+	// If sym is a locally defined generic type, we need the suffix to
+	// stay at the end after mangling so that types/fmt.go can strip it
+	// out again when writing the type's runtime descriptor (#54456).
+	base, suffix := types.SplitVargenSuffix(sym.Name)
+
 	var buf strings.Builder
-	buf.WriteString(sym.Name)
+	buf.WriteString(base)
 	buf.WriteByte('[')
 	for i, targ := range dict.targs {
 		if i > 0 {
@@ -791,6 +796,7 @@ func (dict *readerDict) mangle(sym *types.Sym) *types.Sym {
 		buf.WriteString(targ.LinkString())
 	}
 	buf.WriteByte(']')
+	buf.WriteString(suffix)
 	return sym.Pkg.Lookup(buf.String())
 }