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
3 files changed