chore: improve fork comment and use v0.12.2-preview llgo tag

Update download-llgo.sh comment to explain why the fork is used
(goplus/llgo v0.12.1 has gogen compilation bugs fixed on main
but not yet released). Switch version tag from v0.12.14 to
v0.12.2-preview across action.yml and all workflow files.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

Author: xgopilot

.github/actions/setup-llcppg/action.yml

@@ -8,8 +8,8 @@ inputs:
     description: "LLVM version to install (e.g. 18)"
     default: "19"
   llgo:
-    description: "LLGo version to download (e.g. v0.12.14)"
-    default: "v0.12.14"
+    description: "LLGo version to download (e.g. v0.12.2-preview)"
+    default: "v0.12.2-preview"
 runs:
   using: "composite"
   steps:

.github/actions/setup-llcppg/download-llgo.sh

@@ -39,7 +39,9 @@ esac
 # Remove 'v' prefix from version if present
 VERSION_NUMBER="${VERSION#v}"
 FILENAME="llgo${VERSION_NUMBER}.${OS}-${ARCH}.tar.gz"
-# Use fork releases while upstream release is not available yet.
+# Temporary: use fork releases because goplus/llgo v0.12.1 has gogen
+# compilation bugs fixed on main but not yet released.
+# TODO: revert to github.com/goplus/llgo once v0.12.2+ is officially released.
 URL="https://github.com/luoliwoshang/llgo/releases/download/${VERSION}/${FILENAME}"
 
 echo "Downloading LLGo ${VERSION} for ${OS}-${ARCH}..."

.github/workflows/end2end.yml

@@ -24,7 +24,7 @@ jobs:
           - ubuntu-latest
           - ubuntu-24.04-arm
         llvm: [19]
-        llgo: [v0.12.14]
+        llgo: [v0.12.2-preview]
         go: [1.23]
       fail-fast: false
     runs-on: ${{matrix.os}}

.github/workflows/gentest.yml

@@ -24,7 +24,7 @@ jobs:
           - ubuntu-latest
           - ubuntu-24.04-arm
         llvm: [19]
-        llgo: [v0.12.14]
+        llgo: [v0.12.2-preview]
         go: [1.23]
       fail-fast: false
     runs-on: ${{matrix.os}}

.github/workflows/go.yml

@@ -25,7 +25,7 @@ jobs:
           - macos-latest
           - ubuntu-latest
         llvm: [19]
-        llgo: [v0.12.14]
+        llgo: [v0.12.2-preview]
         go: [1.23]
       fail-fast: false
     runs-on: ${{matrix.os}}