Allow re-assigning to convenience variables

Allow re-assigning to convenience variables

A customer reported somewhat odd gdb behavior, where re-assigning an
array or string to a convenience variable would yield "Too many array
elements".  A test case is:

    (gdb) p $x = "x"
    (gdb) p $x = "xyz"

This patch fixes the problem by making a special case in the evaluator
for assignment to convenience variables, which seems like the correct
behavior.

Note that a previous patch implemented this for Ada, see commit
f411722cb ("Allow re-assigning to convenience variables").

gdb/ChangeLog
2019-11-14  Tom Tromey  <[email protected]>

	* eval.c (evaluate_subexp_standard) <BINOP_ASSIGN>: Do not pass an
	expected type for the RHS if the LHS is a convenience variable.

gdb/testsuite/ChangeLog
2019-11-14  Tom Tromey  <[email protected]>

	* gdb.base/gdbvars.exp (test_convenience_variables): Add
	regression tests.

Change-Id: I5e66a2d243931a5c43c7af4bc9f6717464c2477e