[llvm-debuginfo-analyzer] Return by reference instead of value.

The following functions are used in the unittest, to access
invalid data detected by the Reader during the debug information
analysis:

- getDebugTags
- getWarningOffsets
- getInvalidLocations
- getInvalidCoverages
- getInvalidRanges
- getLinesZero

Just return a reference to the container with the information.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D138092