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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321190 91177308-0d34-0410-b5e6-96231b3b80d8