LibclangTest: remove libclang-test-* tmp dir reliably

Temporary directories created by two LibclangReparseTest tests -
ReparseWithModule and clang_parseTranslationUnit2FullArgv - remained in
the system temporary directory after running libclangTests, because not
all files and subdirectories created in TestDir were added to set
LibclangParseTest::Files.

Differential Revision: https://reviews.llvm.org/D143415