[analyzer] [NFC] PathDiagnostic: Create PathDiagnosticPopUpPiece

Summary:
This new piece is similar to our macro expansion printing in HTML reports:
On mouse-hover event it pops up on variables. Similar to note pieces it
supports `plist` diagnostics as well.

It is optional, on by default: `add-pop-up-notes=true`.

Extra: In HTML reports `background-color: LemonChiffon` was too light,
changed to `PaleGoldenRod`.

Reviewers: NoQ, alexfh

Reviewed By: NoQ

Subscribers: cfe-commits, gerazo, gsd, george.karpenkov, alexfh, xazax.hun,
             baloghadamsoftware, szepet, a.sidorin, mikhail.ramalho,
             Szelethus, donat.nagy, dkrupp

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60670

llvm-svn: 362014