[clang] [CMake][PGO] Use check targets to generate profdata for PGO builds (PR #77347)

[Tom Stellard via cfe-commits]

tstellar wrote:

My goal right now is to make this simple and easy to use.  I'm not opposed to making something configurable, but it would be nice to have a default that actually helped improve performance, so that a new user or a distribution maintainer could just use the cache file as is.

What about if we tried to improve the existing in tree corpus (which only has hello-world).  @petrhosek Is there anything in your corpus that you can contribute upstream?  Any suggestions for what to put in there?  Maybe a few of the bigger c++ files from in the tree?

https://github.com/llvm/llvm-project/pull/77347