Avoid side effects in expression lexers

I noticed that some of the lexers were calling write_dollar_variable
from the lexer.  This seems like a bad practice, so this patch moves
the side effects into the parsers.

I tested this by re-running gdb.fortran and gdb.modula2; the Pascal
compiler on my machine seems not to work, so I couldn't test
gdb.pascal.

I note that the type-tracking in the Pascal is also incorrect, in that
a convenience variable's type may change between parsing and
evaluation (or even during the course of evaluation).

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* p-exp.y (intvar): Remove global.
	(DOLLAR_VARIABLE): Change type.
	(start): Update.
	(exp): Call write_dollar_variable here...
	(yylex): ... not here.
	* m2-exp.y (DOLLAR_VARIABLE): Change type.
	(variable): Call write_dollar_variable here...
	(yylex): ... not here.
	* f-exp.y (DOLLAR_VARIABLE): Change type.
	(exp): Call write_dollar_variable here...
	(yylex): ... not here.