Replace the "implementation" keyword with "checkpoint". This more accurately

Replace the "implementation" keyword with "checkpoint". This more accurately
describes its function as check against unresolved types and definitions.
This is handy for forcing error messages in assembly test cases or otherwise
ensuring that everything is resolved at that point in the assembly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35410 91177308-0d34-0410-b5e6-96231b3b80d8