[clang-format] refactor the use of the SMDiagnostics in replacement warnings

Summary:
Review comments in {D69854} recommended a simpler approach of creating the SMDiagnostics to remove much of the complexity. (thanks @thakis)

@vlad.tsyrklevich I've rebuilt on both Windows and Linux (running Linux with Address and Undefined sanitizers) over the clang code base

Reviewers: thakis, klimek, mitchell-stellar, vlad.tsyrklevich

Reviewed By: thakis

Subscribers: cfe-commits, thakis, vlad.tsyrklevich

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D69921