build: simplify the ICU include handling

This simplifies the ICU flags handling.  This inlines the use into just
the library case, as the ICU usage is for the standard library targets
only.  It uses CMake to properly add the include paths as a system
search path.  This should fix the Linux builds as the include flags were
being de-duplicated as we are now using CMake more than we were
previously.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -373,30 +373,6 @@ function(_add_variant_c_compile_flags)
     endforeach()
   endif()
 
-  set(ICU_UC_INCLUDE_DIR ${SWIFT_${CFLAGS_SDK}_${CFLAGS_ARCH}_ICU_UC_INCLUDE})
-  if(NOT "${ICU_UC_INCLUDE_DIR}" STREQUAL "" AND
-     NOT "${ICU_UC_INCLUDE_DIR}" STREQUAL "/usr/include" AND
-     NOT "${ICU_UC_INCLUDE_DIR}" STREQUAL "/usr/${SWIFT_SDK_${CFLAGS_SDK}_ARCH_${CFLAGS_ARCH}_TRIPLE}/include" AND
-     NOT "${ICU_UC_INCLUDE_DIR}" STREQUAL "/usr/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_TRIPLE}/include")
-   if(SWIFT_COMPILER_IS_MSVC_LIKE)
-     list(APPEND result -I;${ICU_UC_INCLUDE_DIR})
-   else()
-     list(APPEND result -isystem;${ICU_UC_INCLUDE_DIR})
-   endif()
-  endif()
-
-  set(ICU_I18N_INCLUDE_DIR ${SWIFT_${CFLAGS_SDK}_${CFLAGS_ARCH}_ICU_I18N_INCLUDE})
-  if(NOT "${ICU_I18N_INCLUDE_DIR}" STREQUAL "" AND
-     NOT "${ICU_I18N_INCLUDE_DIR}" STREQUAL "/usr/include" AND
-     NOT "${ICU_I18N_INCLUDE_DIR}" STREQUAL "/usr/${SWIFT_SDK_${CFLAGS_SDK}_ARCH_${CFLAGS_ARCH}_TRIPLE}/include" AND
-     NOT "${ICU_I18N_INCLUDE_DIR}" STREQUAL "/usr/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_TRIPLE}/include")
-   if(SWIFT_COMPILER_IS_MSVC_LIKE)
-     list(APPEND result -I;${ICU_I18N_INCLUDE_DIR})
-   else()
-     list(APPEND result -isystem;${ICU_I18N_INCLUDE_DIR})
-   endif()
-  endif()
-
   set("${CFLAGS_RESULT_VAR_NAME}" "${result}" PARENT_SCOPE)
 endfunction()
 
@@ -1447,6 +1423,9 @@ function(_add_swift_library_single target name)
   endif()
 
   # Set compilation and link flags.
+  target_include_directories(${target} SYSTEM PRIVATE
+    ${SWIFT_${SWIFTLIB_SINGLE_SDK}_${SWIFTLIB_SINGLE_ARCHITECTURE}_ICU_UC_INCLUDE}
+    ${SWIFT_${SWIFTLIB_SINGLE_SDK}_${SWIFTLIB_SINGLE_ARCHITECTURE}_ICU_I18N_INCLUDE})
   target_compile_options(${target} PRIVATE
     ${c_compile_flags})
   target_link_options(${target} PRIVATE