Default to use new swift-driver to run lit tests as legacy swift driver
is deprecated for a long time. The lit tests for legacy drivers are also
disable when running new swift-driver.
Tests that relies on legacy swift driver (for example, tests merge
module) can be enabled on conditions `legacy_swift_driver`. Fix and
split up some existing tests that relies on legacy driver.
This change passes through the -file-compilation-dir flag to the
ClangImporter instance as -ffile-compilation-dir. When used in
conjunction with https://github.com/llvm/llvm-project/pull/67744
this will allow for setting the compilation directory of pcm
files using the -file-compilation-dir flag.
Steven Wu
Richard Howell