[ORC] Add a LambdaSymbolResolver convenience class and docs for SymbolResolver.

This patch adds a LambdaSymbolResolver convenience utility that can create an
orc::SymbolResolver from a pair of function objects that supply the behavior for
the lookupFlags and lookup methods.

This class plays the same role for orc::SymbolResolver as the legacy
LambdaResolver class plays for LegacyJITSymbolResolver, and will replace the
latter class once all ORC APIs are migrated to orc::SymbolResolver.

This patch also adds some documentation for the orc::SymbolResolver class as
this was left out of the original commit.

llvm-svn: 323375