header-checker: Extract build variables into envsetup.sh

This commit extracts LLVM build variables from `build-prebuilts.sh` to
`android/envsetup.sh` so that header-checker developers can load these
variables with:

    source android/envsetup.sh

This commit also removes stale `FORCE_BUILD_LLVM_COMPONENTS`.

Test: source android/envsetup.sh; mm; header-abi-dumper --help
Change-Id: Iba5577ba27f8e73d2601d71f538efdb388c4a208

vndk/tools/header-checker/Documentation/Development.md

@@ -0,0 +1,39 @@
+# Development
+
+## Checkout source tree
+
+To checkout the source tree, run the following commands:
+
+    $ mkdir aosp-clang-tools
+
+    $ cd aosp-clang-tools
+
+    $ repo init \
+          -u persistent-https://android.googlesource.com/platform/manifest \
+          -b clang-tools
+
+    $ repo sync
+
+
+## Build instructions
+
+To build `header-abi-dumper`, `header-abi-linker` and `header-abi-diff`:
+
+    $ OUT_DIR=out \
+          development/vndk/tools/header-checker/android/build-prebuilts.sh
+
+
+## Alternative build instructions
+
+If you have a full AOSP master branch source tree, you may build the tools
+with:
+
+    $ source build/envsetup.sh
+
+    $ lunch aosp_arm64-userdebug
+
+    $ cd development/vndk/tools/header-checker
+
+    $ source android/envsetup.sh
+
+    $ mm

vndk/tools/header-checker/android/build-prebuilts.sh

@@ -14,12 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-export LLVM_BUILD_HOST_TOOLS=true
+source "$(dirname "$0")/envsetup.sh"
-export LLVM_PREBUILTS_VERSION=clang-r365631
-export LLVM_RELEASE_VERSION=9.0.6
-
-# FIXME: Workaround to build bionic versioner in the aosp/clang-tools branch.
-export FORCE_BUILD_LLVM_COMPONENTS=true
 
 if [ -z "${OUT_DIR}" ]; then
     echo "error: Must set OUT_DIR"

vndk/tools/header-checker/android/envsetup.sh

@@ -0,0 +1,19 @@
+#!/bin/bash -ex
+
+# Copyright 2019 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+export LLVM_BUILD_HOST_TOOLS=true
+export LLVM_PREBUILTS_VERSION=clang-r365631
+export LLVM_RELEASE_VERSION=9.0.6