[MCJIT] Fix some inconsistent handling of name mangling inside MCJIT.

This patch moves symbol mangling from findSymbol to getSymbolAddress. The
findSymbol, findExistingSymbol and findModuleForSymbol methods now always take
a mangled name, allowing the 'demangle-and-retry' cruft to be removed from
findSymbol. See http://llvm.org/PR28699 for details.

Patch by James Holderness. Thanks very much James!

llvm-svn: 281238