[c-index-test] Use correct executable path to discover resource directory.

Driver builds resource directory path based on provided executable path.
Instead of string "clang" use actual executable path.

rdar://problem/42699514

Reviewers: nathawes, akyrtzi, bob.wilson

Reviewed By: akyrtzi

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50160

llvm-svn: 338741