[llvm-debuginfo-analyzer] LVDoubleMap to return optional<ValueType> instead of null pointer.

[CarlosAlbertoEnciso]

Extracted from llvm/tools/llvm-debuginfo-analyzer/README.txt
https://reviews.llvm.org/D125783#inline-1294164

The more idiomatic LLVM way to handle this would be to have find
return Optional<ValueType>.

[llvmbot]

@llvm/issue-subscribers-debuginfo

Author: Carlos Alberto Enciso (CarlosAlbertoEnciso)

Extracted from [llvm/tools/llvm-debuginfo-analyzer/README.txt](https://github.com/llvm/llvm-project/blob/main/llvm/tools/llvm-debuginfo-analyzer/README.txt) https://reviews.llvm.org/D125783#inline-1294164

The more idiomatic LLVM way to handle this would be to have find
return Optional<ValueType>.