Add a simple helper function to create a 64-bit integer.

Add a function to combine two 32-bit integers into a 64-bit integer.
There are no calls to this function yet, although a subsequent change
will add some in LLDB.

Reviewers: rnk

Differential Revision: http://reviews.llvm.org/D3941

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209777 91177308-0d34-0410-b5e6-96231b3b80d8