clang-import-test had some dead code. I did the following to eliminate it:

- eliminated error handling for the indirect CompilerInstance, which should 
  never generate an error as it is created;
- added a new test for direct importation; and
- removed an unused implementation of the CompleteType() API.

This brings clang-import-test.cpp and ExternalASTMerge.cpp back to 100% 
coverage on all metrics measured by DLLVM_BUILD_INSTRUMENTED_COVERAGE.

Differential Revision: https://reviews.llvm.org/D35220

llvm-svn: 307600