This fixes a similar deprecation warning on various pieces of code:
> DeprecationWarning: Testing an element's truth value will always return True in future versions. Use specific 'len(elem)' or 'elem is not None' test instead.
> samples = log.find ("samples") or empty_element
Add a new performance-log-coalesce.py tool, which groups together
performance-log address-map entries belonging to the same function
into a single symbol, by filling-in missing base symbol addresses.
The addresses are grouped such that each set of addresses
corresponding to a symbol of the same name, in the same source
file, are given the same (unique, but arbitrary) base address.
See the previous commit for why this is necessary.
This should work fine in most cases, however, for logs produced on
Windows, it can over-coalesce addresses belonging to different C++
lambda-functions in the same source file, since they all seem to be
given the same _FUN symbol name.
Use the new tool as part of the pipeline in performance-log-viewer.
Jehan