parse.y: remove coverage-related code fragments

The code fragments that initializes coverage data were scattered into
both parse.y and compile.c.  parse.y allocated a coverage data, and
compile.c initialize the data.

To remove this cross-cutting concern, this change moves the allocation
from "coverage" function of parse.y to "rb_iseq_new_top" of iseq.c.
For the sake, parse.y just counts the line number of the original source
code, and the number is passed via rb_ast_body_t.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64508 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

node.h

@@ -470,6 +470,7 @@ typedef struct node_buffer_struct node_buffer_t;
 typedef struct rb_ast_body_struct {
     const NODE *root;
     VALUE compile_option;
+    int line_count;
 } rb_ast_body_t;
 typedef struct rb_ast_struct {
     VALUE flags;