compiler: use const rather than constexpr

    
    Fixes bootstrap with a C++98 compiler.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/167762

From-SVN: r269710

gcc/go/gofrontend/MERGE

-80a7f6dae0861a06407a44c501b6346a4abd119c
+a99959e6a4a899cfcc4d46e6b54da15d23c58a14
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/ast-dump.cc

@@ -610,7 +610,7 @@ class Type_dumper
                      const char *tag);
   std::pair<bool, unsigned> lookup(const Type*);
 
-  static constexpr unsigned notag = 0xffffffff;
+  static const unsigned notag = 0xffffffff;
 
  private:
   const Type* top_;