Refactor symbol version assignmnt code.

The code to handle symbol versions is getting tricky and hard to
understand, so it is probably time to simplify it. This patch does
the following.

 - Add `DemangledSyms` variable to SymbolTable so that we don't
   need to pass it around to findDemangled.
 - Define `initDemangledSyms` to initialize the variable lazily.
 - hasExternCpp is removed because we no longer have to initialize
   the map eagerly.
 - scanScriptVersion is split.
 - Comments are updated.

llvm-svn: 287002