[ORC] Add SymbolNameSpec, hoist name mangling into Shared.
Introduce two Shared, ExecutionSession-independent pieces:
- SymbolNameSpec (Shared/SymbolNameSpec.h): a StringRef paired with a
SymbolNameKind (Verbatim, Linker, IR, C) describing the naming level
a symbol name is expressed in. A lightweight, dependency-free value
type, so it can describe interface-name constants in Shared headers.
- Mangler (Shared/Mangler.h, Shared/Mangler.cpp): the linker
name-mangling logic factored out of MangleAndInterner. It holds only
a ManglingMode (no ExecutionSession or SymbolStringPool), so names
can be mangled on the executor side as well as the controller
side. It exposes withMangledNameDo (which avoids allocating when
mangling is a no-op) and a mangledCopy convenience, both dispatching
on the spec's SymbolNameKind.
MangleAndInterner becomes a thin wrapper: it derives a ManglingMode
(from an ABIName / ManglingMode / DataLayout), holds a Mangler, and
interns whatever the Mangler produces. Behavior is unchanged.
This lets lookup utilities eventually accept C / IR source names and
mangle them to linker level on the caller's behalf, rather than
requiring pre-mangled names. The MangleAndInternerTest unit tests are
renamed to ManglerTest and now exercise Mangler directly;
MangleAndInterner is left untested since it is slated for removal.
