c++: rename b4_semantic_type_declare as b4_value_type_declare

This is to match the names used in C and api.value.type, even if the
parser actually defines semantic_type.

* data/c++.m4 (b4_semantic_type_declare): Rename as...
(b4_value_type_declare): this.
* data/variant.hh: Likewise.

data/yacc.c

@@ -143,7 +143,6 @@ m4_define([b4_rhs_value],
           [b4_symbol_value([yyvsp@{b4_subtract([$2], [$1])@}], [$3])])
 
 
-
 ## ----------- ##
 ## Locations.  ##
 ## ----------- ##