Updates the CMake version for Swift and the Swift Benchmarks to 3.19.6.
Updates the docs to reflect this change. Does not modify the required version for building the stdlib.
Cleanup and document the configuration of the library path and rpath.
With SWIFT_BENCHMARK_USE_OS_LIBRARIES, it's now possible to directly
build benchmarks for a target device and run those benchmarks on the
device without building or installing Swift.
It's also possible now to specify an absolute SWIFT_LIBRARY_PATH to be
used as an rpath so installation can be skipped.
Moved `lit` test that runs unit tests for benchmark scripts from `validation-test/Python` to `test/benchmark` directory. Run the tests for benchmark infrustructure with single lit invocation:
````
swift-source$ ./llvm/utils/lit/lit.py -sv ${SWIFT_BUILD_DIR}/test-macosx-x86_64/benchmark
````
Documented the invocation of benchmark infrastructure tests in README.md
When a benchmark is added, it's name will be manually added to CMakeLists and
main.swift.
Adding a benchmark no longer requires discovering a nonobvious script.
If the build works locally, then it will work everywhere else.
Note that generate_harness.py should now be renamed to generate_benchmarks.py,
but I think it's name may be hard-coded somewhere in the compiler tests. Anyway,
this script should go away as soon as someone fixes the cmake configuration to
automatically run gyb.
Add support for running benchmarks by reffering to them by their ordinal number in `Benchmark_Driver`, as is supported by `Benchmark_O`(`Onone`, `Ounchecked`).
Updated documentation to reflect this.
