Trimming down Clang usage, mostly eliminating frameworks from headersearch

[lerno]

The only thing untested here is checking whether #include directives still work as expected, otherwise it's only clang internal code that's touched.

[lerno]

There are unfortunately some larger diffs as I ran code reformat on some files as the formatting was poor and hard to read.

[bvdberg]

Are C macro's still working?

[lerno]

Everything should be working still, this should allow normal #include, but not #include with header maps, including precompiled headers and so on. It’s been a while since I did it, but I know you wanted to keep #include and the other C macros, they should be untouched.

[bvdberg]

I did some quick performance tests and it seems this branch is a bit faster... :) :)