RFC: Stand-alone build support

@tstellar Thanks for working on this!

Not quite, it uses LLVM_BUILD_MAIN_SRC_DIR, which is documented here. The name is confusing and that’s my fault, sorry! In practice, it LLVM_GOOGLE_TEST_SRC_DIR could nicely replace this (in Flang it would be as simple as renaming the variable).

I think that it would be very nice for Flang to align with the rest of the project here. However, a fair few Flang developers do use standalone builds for development and hence support for unit testing is quite important. Also, not sure about Clang, but in Flang a large fraction of testing is done through unit tests. This might change with time, but we are not there yet.

So, would it be OK to include LLVM_GOOGLE_TEST_SRC_DIR as an option for standalone builds? And apologies for joining so late!