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This second instalment uses the infrastructure of the previous patch
to allocate a macro map for each macro expansion and assign a virtual
location to each token resulting from the expansion.

To date when cpp_get_token comes across a token that happens to be a
macro, the macro expander kicks in, expands the macro, pushes the
resulting tokens onto a "token context" and returns a dummy padding
token. The next call to cpp_get_token goes look into the token context
for the next token [which is going to result from the previous macro
expansion] and returns it.  If the token is a macro, the macro expander
kicks in and you know the story.

This patch piggy-backs on that macro expansion process, so to speak.
First it modifies the macro expander to make it create a macro map for
each macro expansion. It then allocates a virtual location for each
resulting token.  Virtual locations of tokens resulting from macro
expansions are then stored on a special kind of context called an...