When using a compilation database (compile_commands.json) in very large
projects, significant delays would occur when changing files --
particularly those that happened to be far down the db. Rather than
iterating over the whole list every time, we now build up a lookup table
based on the tail of the filename (and tail of the directory for
widening searches) and iterate over the much smaller list of compile
commands for files with the given name.
Test metrics (from compile_database_perf/test.sh) show a 90% performance
improvement -- from 25 seconds to 2.5 seconds per run.
There is currently a check that tries to prevent c-flags that contain
'-' in them from being unintentionally split and included in the list of
commands. For example, we wouldn't want "-fno-exceptions " to appear as
"-fno" and "-exceptions ". The way this check was done was by making sure
the last character of the split string was a space.
This meant that the very last option to appear in the compile command
was ignored (as it doesn't end with a space). This fix explicitly skips
the ends-with-space check on the last option in the command-line.
This isn't the best fix. Really we should be using the same
argument-processing rules as a shell would rather than just splitting on
'-'. That's a much larger and more complicated change though.
