Copyright headers in translator source code

See https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/63

We have a confusing set of copyright headers in the translator. I think we should move to a general Khronos licence with perhaps an additional line for each company.

Review the copyright-header discussion and the linked pull request #63 first. Clarify the desired Khronos licence wording and whether company-specific lines are required before identifying the affected translator source files; done means the project has an agreed, consistent header policy and the relevant headers reflect it.

Written by the indexing model from the issue text.