[clang][Frontend] Add missing error handling

Some early errors during the ASTUnit creation were not transferred to the `FailedParseDiagnostic` so when the code in `LoadFromCommandLine` swaps its content with the content of `StoredDiagnostics` they cannot be retrieved by the user in any way.

Reviewed By: andrewrk, dblaikie

Differential Revision: https://reviews.llvm.org/D78658