Bug 1650239 - Allow threads in our Mac clang build r=froydnj

Source history does not give any good clues about why this line was added in the first place. In any case, LLVM trunk currently has build bustage when threads are disabled. We could work around the bustage and/or wait for a fix, but it seems like threads are a good thing to have in general nowadays. Maybe this could help with LTO build times.

Differential Revision: https://phabricator.services.mozilla.com/D82447

build/build-clang/build-clang.py

@@ -300,7 +300,6 @@ def build_one_stage(cc, cxx, asm, ld, ar, ranlib, libtool,
             cmake_args += [
                 "-DCMAKE_SYSTEM_NAME=Darwin",
                 "-DCMAKE_SYSTEM_VERSION=10.10",
-                "-DLLVM_ENABLE_THREADS=OFF",
                 # Xray requires a OSX 10.12 SDK (https://bugs.llvm.org/show_bug.cgi?id=38959)
                 "-DCOMPILER_RT_BUILD_XRAY=OFF",
                 "-DLIBCXXABI_LIBCXX_INCLUDES=%s" % libcxx_include_dir,