add --remove-flags argument

[wangnangg]

Hi,

When I use Bear to capture cross-platform builds, I encountered a flag (-mabi=...) that will cause clangd to fail. Therefore, I added a Bear argument to add custom filters for flags. Figured it could be useful for others as well. so here it is.

[rizsotto]

Thanks for this @wangnangg . Looks like a useful thing to do... Although I would do it as a post processing step, but I can understand that it's easier to do in a single step.

The build fails with this message:

Traceback (most recent call last):
   File "/home/travis/build/bear/bear", line 915, in <module>
     sys.exit(intercept_build())
   File "/home/travis/build/bear/bear", line 289, in wrapper
     return function(*args, **kwargs)
   File "/home/travis/build/bear/bear", line 322, in intercept_build
     entries = iter(set(itertools.chain(previous, current)))
   File "/home/travis/build/bear/bear", line 836, in load
     for compilation in Compilation.from_db_entry(entry, tools):
   File "/home/travis/build/bear/bear", line 695, in from_db_entry
     return cls.iter_from_execution(execution, tools)
TypeError: iter_from_execution() missing 1 required positional argument: 'flags_filter'

[wangnangg]

Hi, I fixed errors in Linux. For mac, it looks like it's not relevant to this change.