Test: CoverageMapping: use "RUN: FileCheck" command instead of "RUN: cat | Filecheck".

Change the lit RUN commands for 3 tests to use the following pattern
"FileCheck -input-file ..." instead of "cat ... | FileCheck ..." as
suggested by Justin Bogner.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216085 91177308-0d34-0410-b5e6-96231b3b80d8