Use toolchain clang on macOS

Don't use the just-built clang on macOS. macOS does this more "right"
than the Linux build. Linux will sometimes use the just-built
Swift-driver with the just-built clang, but sometimes would use the
system clang instead. macOS uses the toolchain Swift-driver with the
toolchain clang. This is correct, but it means that if we force the
other clang, we'll get mismatched sanitizer runtimes so the ASAN bot
will fail.

Author: etcwilde

utils/build-script-impl

@@ -1663,7 +1663,7 @@ for host in "${ALL_HOSTS[@]}"; do
         CLANG_BIN="$(build_directory_bin ${LOCAL_HOST} llvm)"
     fi
 
-    if [[ -f "${CLANG_BIN}/clang" ]]; then
+    if [[ -f "${CLANG_BIN}/clang" ]] && [[ "${host}" != "macosx-"* ]]; then
       export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
       export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
     fi
@@ -2735,7 +2735,7 @@ for host in "${ALL_HOSTS[@]}"; do
         CLANG_BIN="$(build_directory_bin ${LOCAL_HOST} llvm)"
     fi
 
-    if [[ -f "${CLANG_BIN}/clang" ]]; then
+    if [[ -f "${CLANG_BIN}/clang" ]] && [[ "${host}" != "macosx-"* ]]; then
       export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
       export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
     fi
@@ -3098,7 +3098,7 @@ for host in "${ALL_HOSTS[@]}"; do
         CLANG_BIN="$(build_directory_bin ${LOCAL_HOST} llvm)"
     fi
 
-    if [[ -f "${CLANG_BIN}/clang" ]]; then
+    if [[ -f "${CLANG_BIN}/clang" ]] && [[ "${host}" != "macosx-"* ]]; then
       export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
       export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
     fi