PNaCl: Add ExpandByVal pass for expanding out by-value struct args and results

PNaCl: Add ExpandByVal pass for expanding out by-value struct args and results

This pass expands out the "byval" and "sret" argument attributes.

This will affect the calling conventions for PPAPI under PNaCl (for
passing PP_Var etc. by value), so the PNaCl PPAPI shims will need to
be updated in order to enable this pass by default.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3400
TEST=PNaCl toolchain trybots + GCC torture tests + LLVM test suite + Spec2k

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-llvm.git;a=commit;h=ac21bcb

[Derek Schuff, 2013-04-23 22:47:05]

LGTM

[Mark Seaborn, 2013-04-24 00:03:00]

Committed patchset #3 manually as rac21bcb (presubmit successful).