Changes

[X86] Use a macro to convert X86ISD names to strings in
getTargetNodeName.
Every case in the switch had a string version of themselves. Two of them
had a typo that used : instead of ::
By using a macro we can automate the string creation and avoid the
possibility of typos like this.
This is similar to what is done on the AMDGPU target.

Mass update the MLIR license header to mention "Part of the LLVM
project"
This is an artifact from merging MLIR into LLVM, the file headers are 
now aligned with the rest of the project.