[llvm-cov] Add the project summary to each source file coverage report.

This patch includes the following changes:
- Included header "Code coverage report" and include the date that the report was created.
- Included title (as specified in a command line option, (i.e llvm-cov  -project-title="Simple Test")
- In the summary, list the elf files that the source code file has contributed to.
- Used column heading for "Line No.", "Count No.", Source".

Differential Revision: https://reviews.llvm.org/D23345

llvm-svn: 279628