Extend clang_getCursor() to take a 'relativeDecl' argument (so speed up searching...

Extend clang_getCursor() to take a 'relativeDecl' argument (so speed up searching). Without a 'relativeDecl', the algorithm is n-squared. For example, running the following command on 'Large.m' takes hours without a 'relatvieDecl'.

snaroff% time ../../Debug/bin/c-index-test Large.ast all > Large.out
snaroff% cat Large.m
#import <Cocoa/Cocoa.h>
#import <QuickTime/QuickTime.h>
#import <OpenGL/OpenGL.h>

With a 'relativeDecl', it takes <30 seconds:-)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@84760 91177308-0d34-0410-b5e6-96231b3b80d8