Add an Offset field to the SourceLocation for LookupResult objects.

Summary:
The Offset provides the offset within the function in a SourceLocation struct. This allows us to show the byte offset within a function. We also track offsets within inline functions as well. Updated the lookup tests to verify the offset for functions and inline functions.

0x1000: main + 32 @ /tmp/main.cpp:45

Reviewers: labath, aadsm, serhiy.redko, jankratochvil, xiaobai, wallace, aprantl, JDevlieghere

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74680