Merge pull request #3916 from vedantk/llvm_enable_modules

Merging into master-next.

Enable module builds of llvm/clang by default

Author: adrian-prantl

utils/build-script-impl

@@ -76,6 +76,7 @@ KNOWN_SETTINGS=(
     swift-analyze-code-coverage "not-merged"     "Code coverage analysis mode for Swift (false, not-merged, merged). Defaults to false if the argument is not present, and not-merged if the argument is present without a modifier."
     swift-tools-enable-lto      ""               "enable LTO compilation of Swift tools. *NOTE* This does not include the swift standard library and runtime. Must be set to one of 'thin' or 'full'"
     llvm-enable-lto             ""               "Must be set to one of 'thin' or 'full'"
+    llvm-enable-modules         "1"              "enable building llvm using modules"
     swift-tools-num-parallel-lto-link-jobs ""    "The number of parallel link jobs to use when compiling swift tools"
     llvm-num-parallel-lto-link-jobs ""           "The number of parallel link jobs to use when compiling llvm"
     swift-stdlib-build-type     "Debug"          "the CMake build variant for Swift"
@@ -634,6 +635,7 @@ function set_build_options_for_host() {
                 -DCOMPILER_RT_ENABLE_WATCHOS:BOOL=FALSE
                 -DCOMPILER_RT_ENABLE_TVOS:BOOL=FALSE
                 -DSANITIZER_MIN_OSX_VERSION="${cmake_osx_deployment_target}"
+                -DLLVM_ENABLE_MODULES:BOOL="$(true_false ${LLVM_ENABLE_MODULES})"
             )
             if [[ $(is_llvm_lto_enabled) == "TRUE" ]]; then
                 if [[ $(cmake_needs_to_specify_standard_computed_defaults) == "TRUE" ]]; then