Begin adding docs and IR-level support for the inalloca attribute

The inalloca attribute is designed to support passing C++ objects by
value in the Microsoft C++ ABI.  It behaves the same as byval, except
that it always implies that the argument is in memory and that the bytes
are never copied.  This attribute allows the caller to take the address
of an outgoing argument's memory and execute arbitrary code to store
into it.

This patch adds basic IR support, docs, and verification.  It does not
attempt to implement any lowering or fix any possibly broken transforms.

When this patch lands, a complete description of this feature should
appear at http://llvm.org/docs/InAlloca.html .

Differential Revision: http://llvm-reviews.chandlerc.com/D2173

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197645 91177308-0d34-0410-b5e6-96231b3b80d8
Add stack alignment information for Sparc.

This matches the data in clang which was added by Jakob Stoklund Olesen in
r179596.

Thanks for erikjv on irc for pointing me to the relevant documents:
http://sparc.com/standards/64.psabi.1.35.ps.Z
page 25: Every stack frame must be 16-byte aligned.

http://sparc.com/standards/psABI3rd.pdf
page 3-10: Although the architecture requires only word alignment, software convention and the operating system require every stack frame to be doubleword aligned.

I tried to add a test, but it looks like sparc doesn't implement dynamic stack
realignment. This will be tested in clang shortly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197646 91177308-0d34-0410-b5e6-96231b3b80d8
Fix a buffer overrun detected by AddressSanitizer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197647 91177308-0d34-0410-b5e6-96231b3b80d8