Bug 1748056 - Build libc++ in Windows clang for real. r=firefox-build-system-reviewers,andi

Differential Revision: https://phabricator.services.mozilla.com/D137256

build/build-clang/build-clang.py

@@ -251,7 +251,6 @@ def build_one_stage(
             "-DCMAKE_INSTALL_PREFIX=%s" % inst_dir,
             "-DLLVM_TARGETS_TO_BUILD=%s" % machine_targets,
             "-DLLVM_ENABLE_ASSERTIONS=%s" % ("ON" if assertions else "OFF"),
-            "-DLLVM_TOOL_LIBCXX_BUILD=%s" % ("ON" if build_libcxx else "OFF"),
             "-DLLVM_ENABLE_BINDINGS=OFF",
             "-DLLVM_ENABLE_CURL=OFF",
         ]
@@ -259,6 +258,17 @@ def build_one_stage(
             cmake_args += [
                 "-DCLANG_REPOSITORY_STRING=taskcluster-%s" % os.environ["TASK_ID"],
             ]
+        # libc++ doesn't build with MSVC because of the use of #include_next.
+        if is_final_stage and os.path.basename(cc[0]).lower() != "cl.exe":
+            cmake_args += [
+                "-DLLVM_FORCE_BUILD_RUNTIME=ON",
+                "-DLLVM_TOOL_LIBCXX_BUILD=%s" % ("ON" if build_libcxx else "OFF"),
+                # libc++abi has conflicting definitions between the shared and static
+                # library on Windows because of the import library for the dll having
+                # the same name as the static library. libc++abi is not necessary on
+                # Windows anyways.
+                "-DLLVM_TOOL_LIBCXXABI_BUILD=%s" % ("OFF" if is_windows() else "ON"),
+            ]
         if not is_final_stage:
             cmake_args += [
                 "-DLLVM_ENABLE_PROJECTS=clang;compiler-rt",