runtime/pprof: correctly encode inlined functions in CPU profile

The pprof profile proto message expects inlined functions of a PC
to be encoded in one Location entry using multiple Line entries.
https://github.com/google/pprof/blob/5e96527/proto/profile.proto#L177-L184

runtime/pprof has encoded the symbolization information by creating
a Location for each PC found in the stack trace and including info
from all the frames expanded from the PC using runtime.CallersFrames.
This assumes inlined functions are represented as a single PC in the
stack trace. (https://go-review.googlesource.com/41256)

In the recent years, behavior around inlining and the traceback
changed significantly (e.g. https://golang.org/cl/152537,
https://golang.org/issue/29582, and many changes). Now the PCs
in the stack trace represent user frames even including inline
marks. As a result, the profile proto started to allocate a Location
entry for each user frame, lose the inline information (so pprof
presented incorrect results when inlined functions are involved),
and confuse the pprof tool with those PCs made up for inline marks.

This CL attempts to detect inlined call frames from the stack traces
of CPU profiles, and organize the Location information as intended.
Currently, runtime does not provide a reliable and convenient way to
detect inlined call frames and expand user frames from a given externally
recognizable PCs. So we use heuristics to recover the groups
  - inlined call frames have nil Func field
  - inlined call frames will have the same Entry point
  - but must be careful with recursive functions that have the
    same Entry point by definition, and non-Go functions that
    may lack most of the fields of Frame.

The followup CL will address the issue with other profile types.

Change-Id: I0c9667ab016a3e898d648f31c3f82d84c15398db
Reviewed-on: https://go-review.googlesource.com/c/go/+/204636


Reviewed-by: Keith Randall <khr@golang.org>