Warn if function or variable cannot be implicitly instantiated

Warn if function or variable cannot be implicitly instantiated

With this patch compiler emits warning if it tries to make implicit instantiation
of a template but cannot find the template definition. The warning can be suppressed
by explicit instantiation declaration or by command line options
-Wundefined-var-template and -Wundefined-func-template. The implementation follows
the discussion of http://reviews.llvm.org/D12326.

Differential Revision: http://reviews.llvm.org/D16396

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266719 91177308-0d34-0410-b5e6-96231b3b80d8