[ORC] Coalesce all of ORC's symbol renaming / linkage-promotion utilities into

one SymbolLinkagePromoter utility.

SymbolLinkagePromoter renames anonymous and private symbols, and bumps all
linkages to at least global/hidden-visibility. Modules whose symbols have been
promoted by this utility can be decomposed into sub-modules without introducing
link errors. This is used by the CompileOnDemandLayer to extract single-function
modules for lazy compilation.

llvm-svn: 343257