Complete complete types early when importing types from Clang module DWARF.

This affects -gmodules only.

Under normal operation pcm_type is a shallow forward declaration
that gets completed later. This is necessary to support cyclic
data structures. If, however, pcm_type is already complete (for
example, because it was loaded for a different target before),
the definition needs to be imported right away, too.
Type::ResolveClangType() effectively ignores the ResolveState
inside type_sp and only looks at IsDefined(), so it never calls
ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(),
which does extra work for Objective-C classes. This would result
in only the forward declaration to be visible.

An alternative implementation would be to sink this into Type::ResolveClangType ( https://github.com/llvm/llvm-project/blob/88235812a71d99c082e7aa2ef9356d43d1f83a80/lldb/source/Symbol/Type.cpp#L5809) though it isn't clear to me how to best do this from a layering perspective.

rdar://problem/52134074

Differential Revision: https://reviews.llvm.org/D70415