CMake: fix missing SWIFT_CONCURRENCY_GLOBAL_EXECUTOR value

`SWIFT_CONCURRENCY_GLOBAL_EXECUTOR` is defined in `stdlib/cmake/modules/StdlibOptions.cmake`, which is not included during the first pass of evaluation of the root `CMakeLists.txt`. It is available on subsequent evaluations after the value is stored in CMake cache. This led to subtle bugs, where `usr/lib/swift_static/linux/static-stdlib-args.lnk` didn't contain certain flags on clean toolchain builds, but did contain them in incremental builds.

Not having these autolinking flags in toolchain builds leads to errors when statically linking executables on Linux.

Author: MaxDesiatov

CMakeLists.txt

@@ -202,6 +202,10 @@ set(SWIFT_HOST_VARIANT_ARCH "${SWIFT_HOST_VARIANT_ARCH_default}" CACHE STRING
 # This is primarily to support building smaller or faster project files.
 #
 
+# Subsequent options may refer to `StdlibOptions`, which have to be defined first.
+list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/stdlib/cmake/modules)
+include(StdlibOptions)
+
 option(SWIFT_APPEND_VC_REV
   "Embed the version control system revision in Swift"
   TRUE)