Differentiation: import PlatformInfo

We did not import this module which defines the crucial variables
required to compute the install paths. As a result, the installation was
corrupted.

Author: compnerd

Runtimes/Supplemental/Differentiation/CMakeLists.txt

@@ -45,6 +45,7 @@ include(GNUInstallDirs)
 include(AvailabilityMacros)
 include(EmitSwiftInterface)
 include(InstallSwiftInterface)
+include(PlatformInfo)
 include(gyb)
 include(ResourceEmbedding)
 include(CatalystSupport)