Setting the documentation heading for #pragma unroll, which should not be with the heading for #pragma clang loop.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213862 91177308-0d34-0410-b5e6-96231b3b80d8
Improving the "integer constant too large" diagnostics based on post-commit feedback from Richard Smith. Amends r213657.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213865 91177308-0d34-0410-b5e6-96231b3b80d8