[Platform] Unify Linux/FreeBSD modulemaps with gyb

A great deal of duplication exists across the Linux and FreeBSD
modulemaps. An Android modulemap would add even more duplication.

Use gyb to conditionally include certain exports for certain
platforms. Most of glibc.modulemap is common across Linux and FreeBSD,
and using gyb we can conditionally include the two exports that differ.

Author: modocache

stdlib/public/Platform/CMakeLists.txt

@@ -35,11 +35,6 @@ foreach(sdk ${SWIFT_SDKS})
       set(arch_subdir "${SWIFT_SDK_${sdk}_LIB_SUBDIR}/${arch}")
       set(module_dir "${SWIFTLIB_DIR}/${arch_subdir}")
 
-      set(glibc_modulemap_source "glibc.${SWIFT_SDK_${sdk}_TRIPLE_NAME}.modulemap.in")
-      if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/${glibc_modulemap_source}")
-        set(glibc_modulemap_source "glibc.modulemap.in")
-      endif()
-
       # Determine the location of glibc headers based on the target.
       set(GLIBC_INCLUDE_PATH "/usr/include")
       set(GLIBC_ARCH_INCLUDE_PATH ${GLIBC_INCLUDE_PATH})
@@ -55,11 +50,24 @@ foreach(sdk ${SWIFT_SDKS})
 
       # Configure the module map based on the target. Each platform needs to
       # reference different headers, based on what's available in their glibc.
+      set(glibc_modulemap_gyb_source "glibc.modulemap.in.gyb")
+      set(glibc_modulemap_gyb_configured
+          "${CMAKE_CURRENT_BINARY_DIR}/${arch_subdir}/glibc.modulemap.gyb")
+      configure_file(
+          ${glibc_modulemap_gyb_source}
+          ${glibc_modulemap_gyb_configured}
+          @ONLY)
+
+      set(gyb_tool "${SWIFT_SOURCE_DIR}/utils/gyb")
       set(glibc_modulemap_configured
           "${CMAKE_CURRENT_BINARY_DIR}/${arch_subdir}/glibc.modulemap")
-      configure_file(${glibc_modulemap_source} ${glibc_modulemap_configured} @ONLY)
-
       add_custom_command_target(glibc_modulemap_target
+          # We can't use handle_gyb_sources() here because we need to ensure
+          # gyb is invoked before we copy glibc.modulemap into the module dir.
+          # FIXME: Instead of using a combination of configure_file() and gyb,
+          #        we should use just gyb--it'd be simpler.
+          COMMAND
+              ${gyb_tool} "-DCMAKE_SDK=${sdk}" -o ${glibc_modulemap_configured} ${glibc_modulemap_gyb_configured}
           COMMAND
               ${CMAKE_COMMAND} -E make_directory ${module_dir}
           COMMAND