Implement Paste

Implement Paste

Add a paste operator '#' to take two identifier-like strings and joint
them.  Internally paste gets represented as a !strconcat() with any
necessary casts to string added.

This will be used to implement basic for loop functionality as in:

for i = [0, 1, 2, 3, 4, 5, 6, 7] {
  def R#i : Register<...>
}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142525 91177308-0d34-0410-b5e6-96231b3b80d8