[llvm/cmake] Replace CMAKE_SOURCE_DIR with PROJECT_SOURCE_DIR

When adding llvm to a build with add_subdirectory, CMAKE_SOURCE_DIR
refers to the source directory of the parent project. We want to use
PROJECT_SOURCE_DIR instead.

Differential Revision: https://reviews.llvm.org/D141521

GitOrigin-RevId: 71c837f6d7d74e44f156ac60a47b1198727ba90f
5 files changed