DebugInfo: ensure negative enumerators are emitted as such

There's still a problem here - since we're not appropriately using the
signedness/range of the enum to chooset the encoding and emission of
enumerators, but GCC has some bugs around this too so I assume that's
not /such/ a high priority though I may get to it soon out of
completeness.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184695 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid adding entries to the DeclContext lookup table multiple times when lazily
constructing a lookup table.

Previously, buildLookup would add lookup table entries for each item lexically
within the DC, and adding the first entry with a given name would trigger the
external source to add all its entries with that name. Then buildLookup would
carry on and re-add those entries all over again.

Instead, follow a simple rule: a declaration from an external source is only
ever made visible by the external source. One exception to this: since we don't
usually build a lookup table for the TU in C, and we never serialize one, we
don't expect the external source to provide lookups in the TU in C, so we build
those ones ourselves.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184696 91177308-0d34-0410-b5e6-96231b3b80d8