Make DiagnosticIDs::getAllDiagnostics use std::vector. NFC.

The size of the result vector is currently around 4600 with
Flavor::WarningOrError, which makes std::vector a better candidate than
llvm::SmallVector.

Patch by: Andras Leitereg!

Differential Revision: https://reviews.llvm.org/D39372

llvm-svn: 321190