Improve location of error messages around identifiers.

Lexer::ReadIdent() now sets last_token_ before returning, like
Lexer::ReadEvalString() does. So all "expected identifiers" and things
that call ReadIdent (pool parser, rule parser, let parser, code parsing
the rule name after a : in a build line) now point the "^ near here" at
what was there instead of the previous last_token

According to manifest_parser_perftest, this is perf-neutral.
3 files changed