PNaCl: Update LLVM revision in pnacl/DEPS
PNaCl: Update LLVM revision in pnacl/DEPS

This pulls in the following LLVM changes:

a4dba98: (mseaborn@chromium.org) PNaCl ABI checker: Reject the "addrspace" attribute
51c0650: (mseaborn@chromium.org) PNaCl: Enable RewritePNaClLibraryCalls and fix it to remove unused decls

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3343
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3429
TEST=PNaCl toolchain trybots
TBR=dschuff@chromium.org

Review URL: https://codereview.chromium.org/16309005

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11471 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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