[llvm-cov] Skip unnecessary coverage computations for "export -summary-only".

Summary:
This speeds up export "summary-only" execution by an order of magnitude or two,
depending on number of threads used for prepareFileReports execution.

Also includes minor refactoring for splitting render of summary and detailed data
in two independent methods.

Reviewers: vsk, morehouse

Reviewed By: vsk

Subscribers: llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D42000

llvm-svn: 322397