cpplib: Remove lexer_pos.

[Per Bothner]

Neil Booth wrote:

>This patch is a minor cleanup that removes an unnecessary data
>structure.  Since keeping line + col in a struct implies bloat of
>cpp_token because alignment issues, there seems little point in
>keeping them together.  Existing uses were mostly redundant, too.
>
Let me repeat my old suggestion:

struct linecol {
  int line : 22;
  /* 1-origin column number. 0 means unknown, not applicable,
       entire line, orlarger than 1023. */
  int column :10;
};

The same struct should be used in decl-nodes, line notes, and replace the
global "line" variable.