We intend to make PdbAstBuilder abstract and implement
PdbAstBuilderClang along with any other languages that wish to use
PDBs. This is the first step.
auto x = ...; return x; doesn't add any value. I'd just return the expression.
Should the return value become Expected<clang::DeclContext>?
The two statements above get repeated several times in this patch. Consider making a helper function like GetTranslationUnitDeclContext.
OK. I brought it up because the goal seems to be to take a step toward making these APIs more general and thus abstractable, and tweaking the return value would seem (to me) to be part of that. If you're planning to make those kinds in a future patch, then I'm satisfied.