Teach c-index-test to reparse the source file a few times when testing

clang_getCursor() via -cursor-at=??? and CINDEXTEST_EDITING is set in
the environment. This mirrors how we test code completion and
source-loading in the presence of this environment variable.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120392 91177308-0d34-0410-b5e6-96231b3b80d8