When looking for symbols, find load addresses in a more robust way.

IRExecutionUnit previously replicated a bunch of logic that already
existed elsewhere for the purpose of getting a load address for a
symbol.  This approach failed to resolve certain types of symbols.
Instead, we now use functions on SymbolContext to do the address
resolution.

This is a cleanup of IRExecutionUnit::FindInSymbols, and also fixes a
latent bug where we looked at the wrong SymbolContext to determine
whether or not it is external.

<rdar://problem/24770829>

llvm-svn: 261704