cmd/compile: emit string symbols during walk

This avoids needing a mutex to protect stringsym,
and preserves a consistent ctxt.Data ordering
in the face of a concurrent backend.

Updates #15756

Change-Id: I775daae11db5db1269533a00f5249e3a03086ffc
Reviewed-on: https://go-review.googlesource.com/40509
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/walk.go

@@ -502,6 +502,8 @@ opswitch:
 	case OTYPE, ONAME, OLITERAL:
 		// TODO(mdempsky): Just return n; see discussion on CL 38655.
 		// Perhaps refactor to use Node.mayBeShared for these instead.
+		// If these return early, make sure to still call
+		// stringsym for constant strings.
 
 	case ONOT, OMINUS, OPLUS, OCOM, OREAL, OIMAG, ODOTMETH, ODOTINTER,
 		OIND, OSPTR, OITAB, OIDATA, OADDR:
@@ -1653,6 +1655,11 @@ opswitch:
 	}
 	if n.Op == OLITERAL {
 		n = typecheck(n, Erv)
+		// Emit string symbol now to avoid emitting
+		// any concurrently during the backend.
+		if s, ok := n.Val().U.(string); ok {
+			_ = stringsym(s)
+		}
 	}
 
 	updateHasCall(n)