[analyzer] [NFC] Move methods for dumping the coverage in HTMLDiagnostics into the class
[analyzer] [NFC] Move methods for dumping the coverage in HTMLDiagnostics into the class

Differential Revision: https://reviews.llvm.org/D51513

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@341724 91177308-0d34-0410-b5e6-96231b3b80d8

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