Simplify LocateDSYMInVincinityOfExecutable by moving

some redundant code into a separate function, 
LookForDsymNextToExecutablePath, and having that function
also look for .dSYM.yaa files in addition to .dSYM
bundles.

Differential Revision: https://reviews.llvm.org/D53305

<rdar://problem/40406580> 

llvm-svn: 344626