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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343257 91177308-0d34-0410-b5e6-96231b3b80d8