…oid truncation

When generating stacks, the runtime automatically expands inline
functions to inline all inline frames in the stack. However, due to the
stack size limit, the final frame may be truncated in the middle of
several inline frames at the same location.

As-is, we assume that the final frame is a normal function, and emit and
cache a Location for it. If we later receive a complete stack frame, we
will first use the cached Location for the inlined function and then
generate a new Location for the "caller" frame, in violation of the
pprof requirement to merge inlined functions into the same Location.

As a result, we:

1. Nondeterministically may generate a profile with the different stacks
combined or split, depending on which is encountered first. This is
particularly problematic when performing a diff of profiles.

2. When split stacks are generated, we lose the inlining information.

We avoid both of these problems by performing a second expansion of the
last stack frame to recover additional inline frames that may have been
lost. This expansion is a bit simpler than the one done by the runtime
because we don't have to handle skipping, and we know that the last
emitted frame is not an elided wrapper, since it by definition is
already included in the stack.

Fixes #37447

Change-Id: If3ca2af25b21d252cf457cc867dd932f107d4c61
Reviewed-on: https://go-review.googlesource.com/c/go/+/221577
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
(cherry picked from commit fadbf74)
Reviewed-on: https://go-review.googlesource.com/c/go/+/222762