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

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

llvm-svn: 209777