[Clang][CGM] style cleanups NFC

Summary:
Forked from:
https://reviews.llvm.org/D80242

Use the getter for access to DebugInfo consistently.
Use break in switch in CodeGenModule::EmitTopLevelDecl consistently.

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: cfe-commits, srhines

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80840