[Perf] Don't get diagnostics if ignoreCompilerErrors is set

[Gerrit0]

Problem

When running typedoc --ignoreCompilerErrors --out docs src/ on the TypeDoc repo, 23% of the time is spent calling getSemanticDiagnostics. The output of this function is used (all errors are printed), but it doesn't make a difference in the generated documentation, and would be a major speed boost if removed.

Suggested Solution

Only call getSemanticDiagnostics if ignoreCompilerErrors is not specified.

[Gerrit0]

Released in v0.15.6