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

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

This patch is a spin-off from https://reviews.llvm.org/D23922. It extends the text view to preserve the same feature as the html view.

Differential Revision: https://reviews.llvm.org/D24241

llvm-svn: 280756