PNaCl: Update LLVM revision in pnacl/DEPS: pexe format changes
PNaCl: Update LLVM revision in pnacl/DEPS: pexe format changes

This pulls in the following LLVM changes:

afa589d: (mseaborn@chromium.org) PNaCl wire format: Use FORWARDTYPEREF consistently for all forward references
bb186d9: (mseaborn@chromium.org) PNaCl wire format: Remove the METADATA_BLOCK block from pexes

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3507
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3518
TEST=PNaCl toolchain trybots
TBR=jvoung@chromium.org

Review URL: https://codereview.chromium.org/17894009

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11618 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client