Set LLVM's git branch to `main` in manifest. (#693)

LLVM renamed their default branch to main from master.
This breaks builds of the SDK from the sources.

This commit retargets LLVM builds to use the git branch main when appropriate.
The version name "master" remains the same to maintain compatibility with existing build scripts

Fixes #692.
diff --git a/emsdk_manifest.json b/emsdk_manifest.json
index 4a613b9..bbc2fe8 100644
--- a/emsdk_manifest.json
+++ b/emsdk_manifest.json
@@ -5,7 +5,7 @@
     "version": "master",
     "bitness": 32,
     "install_path": "llvm/git",
-    "git_branch": "master",
+    "git_branch": "main",
     "url": "https://github.com/llvm/llvm-project.git",
     "custom_install_script": "build_llvm",
     "only_supports_wasm": true,
@@ -19,7 +19,7 @@
     "version": "master",
     "bitness": 64,
     "install_path": "llvm/git",
-    "git_branch": "master",
+    "git_branch": "main",
     "url": "https://github.com/llvm/llvm-project.git",
     "custom_install_script": "build_llvm",
     "only_supports_wasm": true,