gdb: Guarantee that an SAL's end is right before the next statement

When examining a failure that happens when testing
gdb.python/py-symtab.c with clang, I noticed that it was going wrong
because the test assumed that whenever we get an SAL, its end would
always be right before statement in the line table. This is true for GCC
compiled binaries, since gcc only adds statements to the line table, but
not true for clang compiled binaries.

This is the second time I run into a problem where GDB doesn't handle
non-statement line table entries correctly. The other was eventually
committed as 9ab50efc: "gdb: fix until
behavior with trailing !is_stmt lines", but that commit only changes the
behavior for the 'until' command. In this patch I propose a more general
solution, making it so every time we generate the SAL for a given pc, we
set the end of the SAL to before the next statement or the first
instruciton in the next line, instead of naively assuming that to be the
case.

With this new change, the edge case is removed from the processing of
the 'until' command without regressing the accompanying test case, and
no other regressions were observed in the testsuite.

Approved-By: Tom Tromey <tom@tromey.com>