Previously, many error messages inconsistantly used either lexlineno
and lineno.  In general this works out okay because they're almost
always the same.  The only exceptional case is after lexing a
multi-line raw string literal, where lineno will be the line number of
the opening quote and lexlineno is the line number of the closing
quote.

This CL makes the compiler's error message more consistent:

- Lexer error messages related to invalid byte sequences (i.e., NUL
bytes, bad UTF-8 sequences, and non-initial BOMs) are emitted at
lexlineno (i.e., the source line that contains the invalid byte
sequence).

- All other error messages (notably the parser's "syntax errors") now
use lineno.  The minor change from this is that bogus input like:

    package `
    bogus`

will emit "syntax error: unexpected string literal, expecting name"
error at line 1, instead of line 2.

- Instead of maintaining prevlineno all the time, just record it
when/where actually needed and not already available elsewhere (which
turns out to be just one function).

- Lastly, we remove the legacy "syntax error near ..." fallback in
Yerror, now that the parser always emits more detailed syntax error
messages.

Change-Id: Iaf5f784223d0385fa3a5b09ef2b2ad447feab02f
Reviewed-on: https://go-review.googlesource.com/19925Reviewed-by: Robert Griesemer <gri@golang.org>