Added a comment about the launch_bounds attribute's AST node being required. Since there were no test cases for the attribute, some have been added. This promptly demonstrated a bug with the semantic handling, which is also fixed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197637 91177308-0d34-0410-b5e6-96231b3b80d8