compiler: use const rather than constexpr

Fixes bootstrap with a C++98 compiler.

Change-Id: Ifcb52d205f4e3f9bdb3771126443cecff5b66aa2
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/167762
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1 file changed