gccrs: Add edition separation for keywords

gccrs: Add edition separation for keywords

It might be required in the future to get only the keywords from a
specific edition. To do so we need a mean to differentiate keywords based
on their edition. This commit changes the existing keyword macro to
allow such behavior.

gcc/rust/ChangeLog:

	* lex/rust-token.h (enum PrimitiveCoreType): Change enum macro calls.
	(RS_TOKEN_KEYWORD): Remove generic token keyword macro.
	(RS_TOKEN_KEYWORD_2015): Introduce keywords for edition 2015.
	(RS_TOKEN_KEYWORD_2018): Likewise with edition 2018.
	* lex/rust-token.cc (RS_TOKEN_KEYWORD): Remove old macro definition.
	(RS_TOKEN_KEYWORD_2015): Replace with 2015 definition...
	(RS_TOKEN_KEYWORD_2018): ... and 2018 definition.
	* util/rust-keyword-values.cc (RS_TOKEN_KEYWORD):  Likewise.
	(RS_TOKEN_KEYWORD_2015): Likewise.
	(RS_TOKEN_KEYWORD_2018): Likewise.
	* util/rust-keyword-values.h (RS_TOKEN_KEYWORD): Likewise.
	(RS_TOKEN_KEYWORD_2015): Likewise.
	(RS_TOKEN_KEYWORD_2018): Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>