This commit bulk updates all '.h', '.cl', '.inc', and '.cpp' files to
add any missing license headers.
The remaining files are generally CMake, SOURCES, scripts, markdown,
etc.
There are still some '.ll' files which may benefit from a license
header. I can't find an example of an LLVM IR file with a license header
in the rest of LLVM, but unlike most other (sub)projects, libclc has
examples of LLVM IR as source files, compiled and built into the
library.
