[clang] [clang] Forward `-fvalidate-ast-input-files-content` when loading an AST dump (PR #196298)

[Arseniy Zaostrovnykh via cfe-commits]

necto wrote:

I recommend review by commit (first confirms status quo with a test, second adds the fix and demonstrates its effect on the test)

@steakhal @NagyDonat 
This change is to Clang Frontend in general, not just CSA. As you can see from the test case, it improves usability of the CSA CTU mode, and seems to me inconsequential for all users of ASTUnits. If you think we need a review from broader Clang community, I would appreciate a suggestion who else I should ask.

https://github.com/llvm/llvm-project/pull/196298