Bug 1770302 - Remove build_libcxx. r=firefox-build-system-reviewers,andi

Bug 1744890 moved build_libcxx out of the individual json configs, and
bug 1748056 made it always True. We can now remove it.

Differential Revision: https://phabricator.services.mozilla.com/D146873

build/build-clang/README

@@ -38,7 +38,6 @@ build-clang.py accepts a JSON config format with the following fields:
 * ld: Path to the linker.
 * patches: Optional list of patches to apply.
 * build_type: The type of build to make.  Supported types: Release, Debug, RelWithDebInfo or MinSizeRel.
-* build_libcxx: Whether to build with libcxx.  The default is false.
 * build_clang_tidy: Whether to build clang-tidy with the Mozilla checks imported.  The default is false.
 * osx_cross_compile: Whether to invoke CMake for OS X cross compile builds.
 * assertions: Whether to enable LLVM assertions.  The default is false.

build/build-clang/build-clang.py

@@ -192,7 +192,6 @@ def build_one_stage(
     src_dir,
     stage_dir,
     package_name,
-    build_libcxx,
     osx_cross_compile,
     build_type,
     assertions,
@@ -243,7 +242,7 @@ def build_one_stage(
         # 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_TOOL_LIBCXX_BUILD=%s" % ("ON" if build_libcxx else "OFF"),
+                "-DLLVM_TOOL_LIBCXX_BUILD=ON",
                 # 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
@@ -575,11 +574,6 @@ def main():
                 "We only know how to do Release, Debug, RelWithDebInfo or "
                 "MinSizeRel builds"
             )
-    build_libcxx = True
-    if "build_libcxx" in config:
-        build_libcxx = config["build_libcxx"]
-        if build_libcxx not in (True, False):
-            raise ValueError("Only boolean values are accepted for build_libcxx.")
     build_wasm = False
     if "build_wasm" in config:
         build_wasm = config["build_wasm"]
@@ -768,7 +762,6 @@ def main():
             llvm_source_dir,
             stage1_dir,
             package_name,
-            build_libcxx,
             osx_cross_compile,
             build_type,
             assertions,
@@ -796,7 +789,6 @@ def main():
             llvm_source_dir,
             stage2_dir,
             package_name,
-            build_libcxx,
             osx_cross_compile,
             build_type,
             assertions,
@@ -825,7 +817,6 @@ def main():
             llvm_source_dir,
             stage3_dir,
             package_name,
-            build_libcxx,
             osx_cross_compile,
             build_type,
             assertions,
@@ -870,7 +861,6 @@ def main():
             llvm_source_dir,
             stage4_dir,
             package_name,
-            build_libcxx,
             osx_cross_compile,
             build_type,
             assertions,