Bazel fails to respect the environment variables $LLVM_PREBUILTS_VERSION
and $LLVM_RELEASE_VERSION, which are commonly used by LLVM developers
to build using a custom compiler. Fix it.
Bug: 272408039
Change-Id: I3cf18e7814ff52d526017e4fe34b17ab3d8f1080
Most of the variable export code for cc modules can be re-used for
exporting variables for java modules. Refactor this code into a more
composable structure for reuse.
Test: build/bazel/bp2build.sh
Test: manual comparison of
out/soong/soong_injection/cc_toolchain/constants.bzl
with previous output
Change-Id: Ie5a6fee08cc888b7dc69c3e324e5c3f8aa269a8f
