build: honour the libdispatch build type

When configuring libdispatch as part of the swift build on Linux with SourceKit
enabled, we would default to a release build.  However, that results in known
leaks being reported with LSAN when building with a debug standard library.
Pass along the `LIBDISPATCH_BUILD_TYPE` into the CMake build and map that to the
CMAKE_BUILD_TYPE for the project.

utils/build-script-impl

@@ -2132,6 +2132,7 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DSWIFT_EMBED_BITCODE_SECTION:BOOL=$(true_false "${EMBED_BITCODE_SECTION}")
                     -DSWIFT_TOOLS_ENABLE_LTO:STRING="${SWIFT_TOOLS_ENABLE_LTO}"
                     -DSWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER:BOOL=$(true_false "${BUILD_RUNTIME_WITH_HOST_COMPILER}")
+                    -DLIBDISPATCH_CMAKE_BUILD_TYPE:STRING="${LIBDISPATCH_BUILD_TYPE}"
                     "${swift_cmake_options[@]}"
                 )