[docs] Extend the code coverage docs some more
[docs] Extend the code coverage docs some more

Flesh out the section on interpreting coverage reports, mention the
coverage export feature, and add notes on how to build llvm with
coverage turned on.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@281988 91177308-0d34-0410-b5e6-96231b3b80d8

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