Please add FileCheck, not, and lit back into llvm-toolchain

[Scott Constable]

The llvm-toolchain-3.8 change log reads in part:
"removed LLVM-internal tools (lit, FileCheck, not, *-tblgen, etc.);
      not installed by CMake, not intended for distribution"
The "not intended for distribution" argument can easily be made for *-tblgen, but a popular use of clang/LLVM dev packages is to build libtools, and libtools are tested using lit, FileCheck, etc. I'm currently developing two libtools, both of which use lit and FileCheck to run 'make check'. Right now I have to require users to compile LLVM from source on Ubuntu and Debian, which is very inconvenient. The process is easier on MacOS, because home-brew provides lit and FileCheck with the llvm package, as I would expect.

[Scott Constable (sdconsta)]

I'm going to revise this request just a bit. lit actually isn't necessary to include, because it can be trivially obtained with pip, e.g. `pip install lit`. The necessary tools which should be included in llvm-toolchain are at least FileCheck, not, and count. Some users may also need yaml-bench.