Implement clang_getCursorExtent, which provides a source range for the

cursor itself. In particular, for references this returns the source
range of the reference rather than the source range of the thing it
refers to.

Switch c-index-test from clang_getDeclExtent (which will eventually be
deprecated and removed) over to clang_getCursorExtent. The source
ranges we print for references now make sense; fix up the tests
appropriately.

llvm-svn: 93823