Add cross-compilation flags for the newly added Swift source in `lib/ASTGen/`,
similar to how `SwiftCompilerSources/` is already cross-compiled for other
platforms. Make sure the Swift source in the compiler builds and links against
`SWIFTLIB_DIR` in this cross-compilation build directory, not the one that comes
with the native host compiler.
This requires changing the dependency chain in `CROSSCOMPILE` mode, as normally
the Swift compiler is built first when building natively for the host, then it's
used to build the stdlib. However, when cross-compiling the toolchain, the stdlib
must be cross-compiled first by the host compiler, then the portions of the
Swift compiler written in Swift must be cross-compiled with that new stdlib. All
these dependency changes simply change that compilation order when cross-compiling,
including removing the dependency that the Swift compiler is built before the
stdlib when cross-compiling the Swift compiler.
All changes in this pull are gated on the `CROSSCOMPILE` mode, so they will
not affect any of the existing CI or build presets.
Previously, swift-syntax libraries were built in '/CMakeLists.txt', and
were installed in '/lib/CMakeLists.txt'. Consolidate them into
'/lib/SwiftSyntax/CMakeLists.txt'. Now the swift-syntax libraries look
like any other tool libraries outside the 'lib/SwiftSyntax', as they
should.
