[NVPTX] aligned byte-buffers for vector return types

Summary:
Fixes PR21100 which is caused by inconsistency between the declared return type
and the expected return type at the call site. The new behavior is consistent
with nvcc and the NVPTXTargetLowering::getPrototype function.

Test Plan: test/Codegen/NVPTX/vector-return.ll

Reviewers: jholewinski

Reviewed By: jholewinski

Subscribers: llvm-commits, meheff, eliben, jholewinski

Differential Revision: http://reviews.llvm.org/D5612

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220607 91177308-0d34-0410-b5e6-96231b3b80d8
Update the error handling of lib/Linker.

Instead of passing a std::string&, use the new diagnostic infrastructure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220608 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused variable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220610 91177308-0d34-0410-b5e6-96231b3b80d8