[analyzer] Add an option to use the last location in the main source file as the report location.

Previously, we’ve used the last location of the analyzer issue path as the location of the
report. This might not provide the best user experience, when one analyzer a source
file and the issue appears in the header. Introduce an option to use the last location
of the path that is in the main source file as the report location.

New option can be enabled with -analyzer-config report-in-main-source-file=true.

llvm-svn: 182058