[clang][dataflow] Don't analyze templated declarations.

Attempting to analyze templated code doesn't have a good cost-benefit ratio. We
have so far done a best-effort attempt at this, but maintaining this support has
an ongoing high maintenance cost because the AST for templates can violate a lot
of the invariants that otherwise hold for the AST of concrete code. As just one
example, in concrete code the operand of a UnaryOperator '*' is always a prvalue
(https://godbolt.org/z/s3e5xxMd1), but in templates this isn't true
(https://godbolt.org/z/6W9xxGvoM).

Further rationale for not analyzing templates:

* The semantics of a template itself are weakly defined; semantics can depend
  strongly on the concrete template arguments. Analyzing the template itself (as
  opposed to an instantiation) therefore has limited value.

* Analyzing templates requires a lot of special-case code that isn't necessary
  for concrete code because dependent types are hard to deal with and the AST
  violates invariants that otherwise hold for concrete code (see above).

* There's precedent in that neither Clang Static Analyzer nor the flow-sensitive
  warnings in Clang (such as uninitialized variables) support analyzing
  templates.

Reviewed By: gribozavr2, xazax.hun

Differential Revision: https://reviews.llvm.org/D150352

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