Replace the -code-completion-dump option with

Replace the -code-completion-dump option with

  -code-completion-at=filename:line:column

which performs code completion at the specified location by truncating
the file at that position and enabling code completion. This approach
makes it possible to run multiple tests from a single test file, and
gives a more natural command-line interface.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@82571 91177308-0d34-0410-b5e6-96231b3b80d8