Pull request 940 in include-what-you-use: Use `[[noreturn]]` instead of `LLVM_ATTRIBUTE_NORETURN`

[notifi...@include-what-you-use.org]

New pull request 940 by kimgr: Use `[[noreturn]]` instead of `LLVM_ATTRIBUTE_NORETURN`
https://github.com/include-what-you-use/include-what-you-use/pull/940

A recent change to LLVM removed `LLVM_ATTRIBUTE_NORETURN` in favor of plain C++11
`[[noreturn]]`. Since we require C++14 to build these days, just assume that
the `[[noreturn]]` attribute is available.