Infer a reasonable build subdirectory when building a toolchain

build-script-impl effectively ignores the `install_destdir` provided to
it by build-toolchain, and uses something based on the build subdir
instead. But build-script products do follow `install_destdir`, so
make `build-toolchain` provide something closer to correct.

Author: DougGregor

utils/build-toolchain

@@ -56,10 +56,12 @@ case $(uname -s) in
   Darwin)
     SWIFT_PACKAGE=buildbot_osx_package,no_test
     OS_SUFFIX=osx
+    BUILD_SUBDIR=buildbot_osx
   ;;
   Linux)
     SWIFT_PACKAGE=buildbot_linux,no_test
     OS_SUFFIX=linux
+    BUILD_SUBDIR=buildbot_linux
   ;;
   *)
     echo "Unrecognised platform $(uname -s)"
@@ -77,8 +79,10 @@ while [ $# -ne 0 ]; do
     -t|--test)
       if [ "$(uname -s)" == "Linux" ]; then
         SWIFT_PACKAGE=buildbot_linux
+        BUILD_SUBDIR=buildbot_linux
       else
         SWIFT_PACKAGE=buildbot_osx_package
+        BUILD_SUBDIR=buildbot_osx
       fi
   ;;
     --distcc)
@@ -137,7 +141,7 @@ DISPLAY_NAME_SHORT="Local Swift Development Snapshot"
 DISPLAY_NAME="${DISPLAY_NAME_SHORT} ${YEAR}-${MONTH}-${DAY}"
 
 SWIFT_INSTALLABLE_PACKAGE="${RESULT_DIR}/${ARCHIVE}"
-SWIFT_INSTALL_DIR="${RESULT_DIR}/swift-nightly-install"
+SWIFT_INSTALL_DIR="${RESULT_DIR}/${BUILD_SUBDIR}"
 SWIFT_INSTALL_SYMROOT="${RESULT_DIR}/swift-nightly-symroot"
 SWIFT_TOOLCHAIN_DIR="/Library/Developer/Toolchains/${TOOLCHAIN_NAME}.xctoolchain"
 SYMBOLS_PACKAGE="${RESULT_DIR}/${SYM_ARCHIVE}"