[clang-tidy] Fix a crash issue when clang-tidy runs with compilation database.

Summary:
The clang-tidy will trigger an assertion if it's not in the building directory.

TEST:
cd <llvm-repo>/
./build/bin/clang-tidy --checks=-*,modernize-use-nullptr -p build tools/clang/tools/extra/clang-tidy/ClangTidy.cpp

The crash issue is gone after applying this patch.

Fixes PR24834, PR26241

Reviewers: bkramer, alexfh

Subscribers: rizsotto.mailinglist, cfe-commits

Differential Revision: http://reviews.llvm.org/D17335

llvm-svn: 261991