Introduce and use Decl::getAsFunction() to simplify templated function checks

Lift the getFunctionDecl() utility out of the parser into a general
Decl::getAsFunction() and use it to simplify other parts of the implementation.

Reduce isFunctionOrFunctionTemplate() to a simple type check that works the
same was as the other is* functions and move unwrapping of shadowed decls to
callers so it doesn't get run twice.

Shuffle around canSkipFunctionBody() to reduce virtual dispatch on ASTConsumer.
There's no need to query when we already know the body can't be skipped.