build: make SWIFT_BUILD_STDLIB control the stdlib

Permit disabling the standard library build.  This will allow a
toolchain to be built without the standard library now that it is
possible to build just the standard library without the toolchain.

Author: compnerd

CMakeLists.txt

@@ -1049,7 +1049,9 @@ endif()
 # created. This then will cause SwiftSyntax to fail to build.
 #
 # https://bugs.swift.org/browse/SR-5975
-add_subdirectory(stdlib)
+if(SWIFT_BUILD_STDLIB)
+  add_subdirectory(stdlib)
+endif()
 
 if(SWIFT_INCLUDE_APINOTES)
   add_subdirectory(apinotes)