Only set clang linker if it exists

It we're not building clang, the swift driver silently dies and fails to
build. While it shouldn't do that, we should also only tell it to use a
clang that exists. If we're not building clang or the "native" clang
doesn't exist, leave the environment variable unset and let the driver
choose something.

Author: etcwilde

utils/build-script-impl

@@ -1653,8 +1653,11 @@ 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" ]]; 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"
@@ -2722,8 +2725,11 @@ 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" ]]; 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"
@@ -3082,8 +3088,11 @@ 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" ]]; 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