[DWARF] Detect extraction errors in DWARFFormValue::extractValue

Summary:
Although the function had a bool return value, it was always returning
true. Presumably this is because the main type of errors one can
encounter here is running off the end of the stream, and until very
recently, the DataExtractor class made it very difficult to detect that.

The situation has changed now, and we can easily detect errors here,
which this patch does.

Reviewers: dblaikie, aprantl

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77308