Remove stale clang prebuilts and update clang-stable symlink.

Removing clang-3016494.
Removing clang-3217047.
Removing clang-3688880.
Reset clang-stable to point to clang-3859424.

Test: Build aosp/master and verified NDK is actually using
      both clang-3362437 and clang-2812033.
Change-Id: I26404fd47117eb9f863e4f8974340b48002e8301
559 files changed