When diagnosing a problem with C++ code imported from Carbon, include
the location of the specific `import Cpp` statement that imported the
C++ code as part of the backtrace, rather than providing a location in a
generated file that doesn't exist on disk.
Also fix handling in autoupdate of check lines that contain multiple
file name and line number pairs to use the matched file name for
remapping of locations rather than the first file name in the line.