Relicensing code requires ownership of the code in question or some form of legal delegation. For example VLC relicensed something - I forget exactly what, may

Relicensing code requires ownership of the code in question or some form of legal delegation. For example VLC relicensed something - I forget exactly what, maybe a core library - and the guy in charge had to track down every single person who contributed non-trivial amounts of code and obtain their permission to alter the license. To do what you propose would effectively require rewriting Clang from scratch, but even then it might be considered a derivative work and thus beholden to the original copyright holders in some way. (I'm not a lawyer so I'm a little hazy on the specifics.) Or at the very least, you'd have to write enough new or improved code that convinces people to use your fork of Clang, your part of which would be under GPL and so obligate distributors of your fork accordingly.