[clang-import-test] Lookup inside contexts

clang-import-test has until now been only able to report top-level Decls.
This is clearly insufficient; we should be able to look inside structs 
and namespaces also.  This patch adds new test cases for a variety of 
lookups inside existing ASTContexts, and adds the functionality necessar
to make most of these testcases work.  (One testcase is known to fail 
because of ASTImporter limitations when importing templates; I'll look 
into that separately.)

This patch also separates the core functionality out into 
ExternalASTMerger, an interface that allows clients like LLDB to make 
use of it.  clang-import-test now only has the machinery necessary to
set up the tests.

Differential revision: https://reviews.llvm.org/D30435

llvm-svn: 299976