Revert r333147 "[ORC] Add findSymbolIn() wrapper to C bindings."

This reverts r333147 until https://reviews.llvm.org/D47308 is ready to
be reviewed. r333147 exposed a behavioural difference between
OrcCBindingsStack::findSymbolIn() and OrcCBindingsStack::findSymbol(),
where only the latter does name mangling. After r333147 that causes a
test failure on OSX, because the new test looks for main using
findSymbolIn() but the mangled name is _main.

llvm-svn: 333152