build: query the resource dir when needed

When building the standard library standalone with the host compiler, we
do not have the location of the resource dir available to us nor can it
be computed.  Use `-print-resource-dir` to query the value from the
compiler and use that.  This is needed to cross-compile the standard
library to android from Windows.

Author: compnerd

stdlib/public/SwiftShims/CMakeLists.txt

@@ -84,7 +84,15 @@ endif()
 # First extract the "version" used for Clang's resource directory.
 string(REGEX MATCH "[0-9]+\\.[0-9]+(\\.[0-9]+)?" CLANG_VERSION
   "${LLVM_PACKAGE_VERSION}")
-set(clang_headers_location "${LLVM_LIBRARY_OUTPUT_INTDIR}/clang/${CLANG_VERSION}")
+if(NOT SWIFT_INCLUDE_TOOLS AND SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
+  execute_process(COMMAND ${CMAKE_C_COMPILER} -print-resource-dir
+    OUTPUT_VARIABLE clang_headers_location
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_QUIET)
+  message(STATUS "using clang resource directory: ${clang_headers_location}")
+else()
+  set(clang_headers_location "${LLVM_LIBRARY_OUTPUT_INTDIR}/clang/${CLANG_VERSION}")
+endif()
 
 if("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
   set(cmake_symlink_option "copy_directory")