Copy arguments passed by value into explicit allocas for ASan.

ASan determines the stack layout from alloca instructions. Since
arguments marked as "byval" do not have an explicit alloca instruction, ASan
does not produce red zones for them. This commit produces an explicit alloca
instruction and copies the byval argument into the allocated memory so that red
zones are produced.

Patch by Matt Morehouse.

Differential revision: https://reviews.llvm.org/D34789

llvm-svn: 307342