Added initial support to the lldb_private::SymbolFile for finding

namespaces by name given an optional symbol context. I might end up
dressing up the "clang::NamespaceDecl" into a lldb_private::Namespace
class if we need to do more than is currenlty required of namespaces.
Currently we only need to be able to lookup a namespace by name when
parsing expressions, so I kept it simple for now. The idea here is
even though we are passing around a "clang::NamespaceDecl *", that
we always have it be an opaque pointer (it is forward declared inside
of "lldb/Core/ClangForward.h") and we only use clang::NamespaceDecl
implementations inside of ClangASTContext, or ClangASTType when we need
to extract information from the namespace decl object.

llvm-svn: 118737