Ignore template instantiations if not in AsIs mode

Ignore template instantiations if not in AsIs mode

Summary:
IgnoreUnlessSpelledInSource mode should ignore these because they are
not written in the source.  This matters for example when trying to
replace types or values which are templated.  The new test in
TransformerTest.cpp in this commit demonstrates the problem.

In existing matcher code, users can write
`unless(isInTemplateInstantiation())` or `unless(isInstantiated())` (the
user must know which to use).  The point of the
TK_IgnoreUnlessSpelledInSource mode is to allow the novice to avoid such
details.  This patch changes the IgnoreUnlessSpelledInSource mode to
skip over implicit template instantiations.

This patch does not change the TK_AsIs mode.

Note: An obvious attempt at an alternative implementation would simply
change the shouldVisitTemplateInstantiations() in ASTMatchFinder.cpp to
return something conditional on the operational TraversalKind.  That
does not work because shouldVisitTemplateInstantiations() is called
before a possible top-level traverse() matcher changes the operational
TraversalKind.

Reviewers: sammccall, aaron.ballman, gribozavr2, ymandel, klimek

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80961