[clang-import-test] Lookup inside contexts

[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