Improve filtering in default-backtrace-ice.rs.

This test is supposed to ensure that full backtraces are used for ICEs.
But it doesn't actually do that -- the filtering done cannot distinguish
between a full backtrace versus a short backtrace.

So this commit changes the filtering to preserve the existence of
`__rust_{begin,end}_short_backtrace` markers, which only appear in full
backtraces. This change means the test now tests what it is supposed to
test.

Also, the existing filtering included a rule that excluded any line
starting with two spaces. This was too strong because it filtered out
some parts of the error message. (This was not a showstopper). It was
also not strong enough because it didn't work with three digit stack
frame numbers, which just started seeing after upgrading my Ubuntu
distro to 23.04 machine (this *was* a showstopper).

So the commit replaces that rule with two more precise rules, one for
lines with stack frame numbers, and one for "at ..." lines.

tests/ui/panics/default-backtrace-ice.rs

@@ -2,8 +2,20 @@
 // compile-flags:-Z treat-err-as-bug=1
 // error-pattern:stack backtrace:
 // failure-status:101
+// ignore-msvc
 // normalize-stderr-test "note: .*" -> ""
 // normalize-stderr-test "thread 'rustc' .*" -> ""
-// normalize-stderr-test " .*\n" -> ""
+// normalize-stderr-test " +\d+:.*__rust_begin_short_backtrace.*" -> "(begin_short_backtrace)"
+// normalize-stderr-test " +\d+:.*__rust_end_short_backtrace.*" -> "(end_short_backtrace)"
+// normalize-stderr-test " +\d+:.*\n" -> ""
+// normalize-stderr-test " +at .*\n" -> ""
+//
+// This test makes sure that full backtraces are used for ICEs when
+// RUST_BACKTRACE is not set. It does this by checking for the presence of
+// `__rust_{begin,end}_short_backtrace` markers, which only appear in full
+// backtraces. The rest of the backtrace is filtered out.
+//
+// Ignored on msvc becaue the `__rust_{begin,end}_short_backtrace` symbols
+// aren't reliable.
 
 fn main() { missing_ident; }

tests/ui/panics/default-backtrace-ice.stderr

@@ -1,8 +1,13 @@
 error[E0425]: cannot find value `missing_ident` in this scope
+ --> $DIR/default-backtrace-ice.rs:21:13
+ |
 LL | fn main() { missing_ident; }
+ | ^^^^^^^^^^^^^ not found in this scope
 
 
 stack backtrace:
+(end_short_backtrace)
+(begin_short_backtrace)
 
 error: the compiler unexpectedly panicked. this is a bug.