[TSan] Ensure libc++ can be linked when built with TSan

Required change to make "Swift CI preset for LLVM libdispatch tests" [1]
work.

This goes the opposite direction of [2], ensuring that the link step
succeeds for the prodcued dynamic library (*.so). In the future versions
of compiler-rt this will not be necessary since we will create a static
archive instead.

[1] swiftlang/swift#24330
[2] https://reviews.llvm.org/D58013

Author: yln

lib/tsan/CMakeLists.txt

@@ -248,7 +248,7 @@ if(COMPILER_RT_LIBCXX_PATH AND
     set(LIBCXX_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/libcxx_tsan_${arch})
     add_custom_libcxx(libcxx_tsan_${arch} ${LIBCXX_PREFIX}
       DEPS ${TSAN_RUNTIME_LIBRARIES}
-      CFLAGS ${TARGET_CFLAGS} -fsanitize=thread
+      CFLAGS ${TARGET_CFLAGS} -fsanitize=thread ${COMPILER_RT_LIBDISPATCH_CFLAGS}
       USE_TOOLCHAIN)
     list(APPEND libcxx_tsan_deps libcxx_tsan_${arch}-build)
   endforeach()