This is to address llvm#250.
Where build requests were piling up on slower builders because
they were clean/don't clean/clean/ don't clean etc. and we could
not merge those requests because of that difference.

Galina suggested that if we knew that the builder was going to
clean the build first anyway, we could merge them. This commit
implements that.

We do this by first setting a "clean" attribute on the builder's
factory object, that we can find later via in the collapse callback.

So far only ClangBuilder passes this on but any builder can opt
in by copying what it does.

In the collapse function when comparing properties we delete the
"clean_obj" key from both sets of properties if we know that the
builder's factory is always clean.

With some logging (not included in this commit) we can see it now
collapses a clean/non-clean pair of requests:
```
collapseRequests
selfBuildSetProperties: {'scheduler': ('main:clang,clang-tools-extra,compiler-rt,flang,lld,llvm,mlir', 'Scheduler')}
otherBuildSetProperties: {'scheduler': ('main:clang,clang-tools-extra,compiler-rt,flang,lld,llvm,mlir', 'Scheduler'), 'clean_obj': (True, 'Change')}
Removing clean_obj from build set properties.
Build set properties were the same.
```

This has beeen tested by my colleague Omair Javaid on a test build
master. Of course we only tested it with one of Linaro's builders,
so it's possible we see other side effects in production.