Add option to include multiple lines in snippets.

When a diagnostic includes a highlighted range spanning multiple lines, clang
now supports printing out multiple lines of context if necessary to show the
highlighted ranges. This is not yet exposed in the driver, but can be enabled
by "-Xclang -fcaret-diagnostics-max-lines -Xclang N".

This is experimental until we can find out whether it works well in practice,
and if so, what a good default for the maximum number of lines is.

llvm-svn: 303589