[llvm] [DLCov 1/5] Add CMake option to enable enhanced line number coverage tracking (PR #107278)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> I will review the implementation, but I am curious whether we only modified the metadata/`DebugLoc`. To be more precise, my main concern is memory usage increase, and whether we have affected `llvm::Instruction` in any way other than metadata.

There will be increased memory usage - more exactly, we increase the size of `DebugLoc`, _not_ `!DILocation`, which means the extra storage is part of the `Instruction` class. The intent here is that we conditionally compile this extra information in for a specific build configuration designed to test the compiler itself with debugify; in ordinary builds, including debug builds, the extra information is not stored, so run time and memory usage should be unaffected. 

https://github.com/llvm/llvm-project/pull/107278