[Index] Add index::IndexingOptions::IndexImplicitInstantiation

Summary:
With IndexImplicitInstantiation=true, the following case records an occurrence of B::bar in A::foo, which will benefit cross reference tools.

template <class T> struct B { void bar() {}};
template <class T> struct A { void foo(B<T> *x) { x->bar(); }};
int main() { A<int> a; a.foo(0); }

Reviewers: akyrtzi, arphaman, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49002

llvm-svn: 336606