[asan] Copy arguments passed by value into explicit allocas for ASan

  Summary:
  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.

  Submitted on behalf of @morehouse (Matt Morehouse)

  Reviewers: eugenis, vitalybuka

  Reviewed By: eugenis

  Subscribers: hiraditya, llvm-commits

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

llvm-svn: 308387