Summary:
Without this we could silently accept an invalid prologue because the
default DataExtractor behavior is to return an empty string when
reaching the end of file. And empty string is also used to terminate
these lists.

This makes the parsing code slightly more complicated, but this
complexity will go away once the parser starts working with truncating
data extractors. The reason I am doing it this way is because without
this, the truncation would regress the quality of error messages (right
now, we produce bad error messages only near EOF, but truncation would
make everything behave as if it was near EOF).

Reviewers: dblaikie, probinson, jhenderson

Subscribers: hiraditya, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77555