SwiftCore: Cache computed platform info

Implements a similar caching mechanism to what CMake uses for detecting
the compiler. The file is created based on the computed values and
allows us to avoid calling the compiler if we aren't going to extract
out any platform info.

Author: etcwilde

.gitignore

@@ -97,3 +97,4 @@ Runtimes/**/*.inc
 Runtimes/**/*.json
 Runtimes/**/*.modulemap
 Runtimes/**/*.in
+!Runtimes/**/*.cmake.in

Runtimes/Core/cmake/modules/DetectedPlatformInfo.cmake.in

@@ -0,0 +1,8 @@
+set(SwiftCore_SIZEOF_POINTER @SwiftCore_SIZEOF_POINTER@)
+set(SwiftCore_MODULE_TRIPLE @SwiftCore_MODULE_TRIPLE@)
+set(SwiftCore_PLATFORM_SUBIDR @SwiftCore_PLATFORM_SUBIDR@)
+set(SwiftCore_ARCH_SUBDIR @SwiftCore_ARCH_SUBDIR@)
+
+set(SwiftCore_SWIFT_AVAILABILITY_PLATFORM @SwiftCore_SWIFT_AVAILABILITY_PLATFORM@)
+
+set(SwiftCore_PLATFORM_INFO_SET 1)

Runtimes/Core/cmake/modules/PlatformInfo.cmake

@@ -1,3 +1,9 @@
+include("${PROJECT_BINARY_DIR}/build/DetectedPlatformInfo.cmake" OPTIONAL)
+
+if(SwiftCore_PLATFORM_INFO_SET)
+  return()
+endif()
+
 if(NOT SwiftCore_SIZEOF_POINTER)
   set(SwiftCore_SIZEOF_POINTER "${CMAKE_SIZEOF_VOID_P}" CACHE STRING "Size of a pointer in bytes")
   message(CONFIGURE_LOG "Stdlib Pointer size: ${CMAKE_SIZEOF_VOID_P}")
@@ -80,3 +86,7 @@ if(SwiftCore_VARIANT_MODULE_TRIPLE)
     message(WARNING "Unknown platform ${platform} for variant availability")
   endif()
 endif()
+
+configure_file("${CMAKE_CURRENT_LIST_DIR}/DetectedPlatformInfo.cmake.in"
+  "${PROJECT_BINARY_DIR}/build/DetectedPlatformInfo.cmake"
+  @ONLY)