Remove the relative path hack in scan-build-py that converts a fully qualified directory name and a fully qualified file path to a relative path before running the analyzer on a file.
This hack is not needed: the bad interaction with SATestsBuild.py it was intended to address is actually the same underlying problem that D24163 fixed. Further, because the hack would always relativize paths, it caused SATestBuild.py to be unable to properly line up issues when the build system changed directory and then built a source file in a child directory but used a fully-qualified path for the source file.
Laszlo: Are you OK with me removing this hack? This is the last outstanding issue in scan-build-py preventing us in using it for our regression suite.
Note: I still think the approach used here (and especially in intercept-build) is not ideal. I really think we should be storing both the working directory and the actual path argument (even if it is relative) in the compilation database. Storing the fully-qualified paths loses information (i.e., which of the many possible paths for that file was actually used) and makes replaying the build with fidelity impossible. That said, after D24163 I don't have any examples where is actually an issue in practice.