add PGO to llvm/clang

[galdutro13]

There are some packages that I've managed to build with PGO using clang that with gcc the profiling phase happens to be incorrect and result in worse performance. An example is webkit-gtk where I've managed to gain about 12 percent more performance in some javascript benchmark like jetstream 2 through clang pgo, where gcc simply gets confused because of the change of directories of the source files that happen during build.
The problem is that you have to run 'llvm-profdata merge" after the data is collected from the instrumented runs, so it also would need to disable sandboxing.