[Stdlib] Add Darwin/Glibc as a dependency for SwiftPrivate.

The changes in #19614 require Darwin/Glibc to build first. This usually happened anyway (and thus the problem wasn't noticed for a while) but sometimes SwiftPrivate would win the race and the build would fail.

rdar://problem/45624328

Author: mikeash

stdlib/private/SwiftPrivate/CMakeLists.txt

@@ -5,6 +5,14 @@ add_swift_target_library(swiftSwiftPrivate ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
   IO.swift
   ShardedAtomicCounter.swift
 
+  SWIFT_MODULE_DEPENDS_IOS Darwin
+  SWIFT_MODULE_DEPENDS_OSX Darwin
+  SWIFT_MODULE_DEPENDS_TVOS Darwin
+  SWIFT_MODULE_DEPENDS_WATCHOS Darwin
+  SWIFT_MODULE_DEPENDS_LINUX Glibc
+  SWIFT_MODULE_DEPENDS_FREEBSD Glibc
+  SWIFT_MODULE_DEPENDS_CYGWIN Glibc
+  SWIFT_MODULE_DEPENDS_HAIKU Glibc
   SWIFT_COMPILE_FLAGS
   INSTALL_IN_COMPONENT stdlib-experimental)