[clang][tooling] Accept custom diagnostic options in ToolInvocation

This patch allows the clients of `ToolInvocation` to provide custom diagnostic options to be used during driver -> cc1 command-line transformation and parsing.

Tests covering this functionality are in a follow-up commit. To make this testable, the `DiagnosticsEngine` needs to be properly initialized via `CompilerInstance::createDiagnostics`.

Reviewed By: dexonsmith, arphaman

Differential Revision: https://reviews.llvm.org/D108976