[lldb][TypeSystemClang] Set access specifier for EnumConstantDecl's (llvm#174865)

LLDB was already setting the access specifier on `EnumDecl`s
unconditionally to `AS_public`. But it wasn't doing so for the
`EnumConstantDecl`s. This triggered a Clang assertion during
auto-completion of expressions
(llvm#171913).

Ideally the code-completion accessibility check would honor the
`AccessControl` language option, but that change is harder to
test/justify. Since this is a pretty straight-forward change I want to
land this before trying to do that.

There is no test coverage for this because it relies on the order in
which Clang decides to check the decls in scope for auto-completion,
which seems fragile. It's also consistent with how we handle access to
other decls in `TypeSystemClang`.

Fixes llvm#171913

Author: Michael137

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -8594,6 +8594,7 @@ clang::EnumConstantDecl *TypeSystemClang::AddEnumerationValueToEnumerationType(
     enumerator_decl->setDeclName(&getASTContext().Idents.get(name));
   enumerator_decl->setType(clang::QualType(enutype, 0));
   enumerator_decl->setInitVal(getASTContext(), value);
+  enumerator_decl->setAccess(AS_public);
   SetMemberOwningModule(enumerator_decl, enum_decl);
 
   if (!enumerator_decl)