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