Thread ExecutionContextScope through GetByteSize where possible (NFC-ish)

This patch has no effect for C and C++. In more dynamic languages,
such as Objective-C and Swift GetByteSize() needs to call into the
language runtime, so it's important to pass one in where possible. My
primary motivation for this is some work I'm doing on the Swift
branch, however, it looks like we are also seeing warnings in
Objective-C that this may resolve.  Everything in the SymbolFile
hierarchy still passes in nullptrs, because we don't have an execution
context in SymbolFile, since SymbolFile transcends processes.

Differential Revision: https://reviews.llvm.org/D84267