constexprify some tree variables

Since we use C++11 by default now, we can
use constexpr for some const decls in tree-core.h.

This patch does that and it allows for better optimizations
of GCC code with checking enabled and without LTO.

For an example generic-match.cc compiling is speed up due
to the less number of basic blocks and less debugging info
produced. I did not check the speed of compiling the same source
but rather the speed of compiling the old vs new sources here
(but with the same compiler base).

The small slow down in the parsing of the arrays in each TU
is migrated by a speed up in how much code/debugging info
is produced in the end.

Note I looked at generic-match.cc since it is one of the
compiling sources which causes parallel building to stall and
I wanted to speed it up.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	PR middle-end/14840
	* tree-core.h (tree_code_type): Constexprify
	by including all-tree.def.
	(tree_code_length): Likewise.
	* tree.cc (tree_code_type): Remove.
	(tree_code_length): Remove.