[Object] Change ObjectFile::getSymbolValue() return type to Expected<uint64_t>

Summary:
In D77860, we have changed `getSymbolFlags()` return type to `Expected<uint32_t>`.
This change helps bubble the error further up the stack.

Reviewers: jhenderson, grimar, JDevlieghere, MaskRay

Reviewed By: jhenderson

Subscribers: hiraditya, MaskRay, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79075