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.

(cherry picked from commit f446c25)

Author: etcwilde

utils/build-script-impl

@@ -1670,8 +1670,11 @@ for host in "${ALL_HOSTS[@]}"; do
     else
         CLANG_BIN="$(build_directory_bin ${LOCAL_HOST} llvm)"
     fi
-    export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
-    export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
+
+    if [[ -f "${CLANG_BIN}/clang" ]] && [[ -z ${ENABLE_ASAN+x} ]]; then
+      export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
+      export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
+    fi
 
     if [[ "${NATIVE_SWIFT_TOOLS_PATH}" ]] ; then
         SWIFTC_BIN="${NATIVE_SWIFT_TOOLS_PATH}/swiftc"
@@ -2782,8 +2785,11 @@ for host in "${ALL_HOSTS[@]}"; do
     else
         CLANG_BIN="$(build_directory_bin ${LOCAL_HOST} llvm)"
     fi
-    export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
-    export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
+
+    if [[ -f "${CLANG_BIN}/clang" ]] && [[ -z ${ENABLE_ASAN+x} ]]; then
+      export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
+      export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
+    fi
 
     if [[ "${NATIVE_SWIFT_TOOLS_PATH}" ]] ; then
         SWIFTC_BIN="${NATIVE_SWIFT_TOOLS_PATH}/swiftc"
@@ -3074,8 +3080,11 @@ for host in "${ALL_HOSTS[@]}"; do
     else
         CLANG_BIN="$(build_directory_bin ${LOCAL_HOST} llvm)"
     fi
-    export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
-    export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
+
+    if [[ -f "${CLANG_BIN}/clang" ]] && [[ -z ${ENABLE_ASAN+x} ]]; then
+      export SWIFT_DRIVER_CLANG_EXEC="${CLANG_BIN}/clang"
+      export SWIFT_DRIVER_CLANGXX_EXEC="${CLANG_BIN}/clang++"
+    fi
 
     # Set the build options for this host
     set_build_options_for_host $host