Add Clang-16 to emsdk manifest, to allow bootstrapping a newer clang with an older clang.

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index d7e8644..b088912 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -1007,6 +1007,17 @@
     cmdline += ['-DCMAKE_OSX_DEPLOYMENT_TARGET=11.0']
     cmdline += [*extra_cmake_args, src_root]
 
+    # If we have a bootstrap compiler package installed, then use that compiler
+    # in the cmake build.
+    for ver in ['16.0.6', '15.0.7']:
+        bootstrap_compiler_path = os.path.abspath(f'llvm/{ver}/build_llvmorg-{ver}_64/bin')
+        bs_clang = os.path.join(bootstrap_compiler_path, 'clang')
+        bs_clangxx = os.path.join(bootstrap_compiler_path, 'clang++')
+        if os.path.isfile(bs_clang) and f'llvm/{ver}' not in build_root:
+            info(f'Using bootstrap compiler from {bs_clang} in CMake configure')
+            cmdline += [f'-DCMAKE_C_COMPILER={bs_clang}', f'-DCMAKE_CXX_COMPILER={bs_clangxx}']
+            break
+
     print(f'Running CMake: {cmdline}')
 
     # Specify the deployment target also as an env. var, since some Xcode versions

emsdk_manifest.json

diff --git a/emsdk_manifest.json b/emsdk_manifest.json
index 9992e7d..0b353b4 100644
--- a/emsdk_manifest.json
+++ b/emsdk_manifest.json
@@ -30,6 +30,36 @@
   },
 
   {
+    "id": "llvm-git",
+    "version": "15.0.7",
+    "bitness": 64,
+    "install_path": "llvm/15.0.7",
+    "git_branch": "llvmorg-15.0.7",
+    "url": "https://github.com/llvm/llvm-project.git",
+    "custom_install_script": "build_llvm",
+    "only_supports_wasm": true,
+    "activated_path": "%installation_dir%/%llvm_build_bin_dir%",
+    "activated_cfg": "LLVM_ROOT='%installation_dir%/%llvm_build_bin_dir%'",
+    "activated_env": "LLVM_ROOT=%installation_dir%/%llvm_build_bin_dir%;EMCC_WASM_BACKEND=1",
+    "cmake_build_type": "Release"
+  },
+
+  {
+    "id": "llvm-git",
+    "version": "16.0.6",
+    "bitness": 64,
+    "install_path": "llvm/16.0.6",
+    "git_branch": "llvmorg-16.0.6",
+    "url": "https://github.com/llvm/llvm-project.git",
+    "custom_install_script": "build_llvm",
+    "only_supports_wasm": true,
+    "activated_path": "%installation_dir%/%llvm_build_bin_dir%",
+    "activated_cfg": "LLVM_ROOT='%installation_dir%/%llvm_build_bin_dir%'",
+    "activated_env": "LLVM_ROOT=%installation_dir%/%llvm_build_bin_dir%;EMCC_WASM_BACKEND=1",
+    "cmake_build_type": "Release"
+  },
+
+  {
     "id": "releases",
     "version": "%releases-tag%",
     "bitness": 64,