Rework the way we handle template instantiation for

implicitly-generated AST nodes. We previously built instantiated nodes
for each of these AST nodes, then passed them on to Sema, which was
not prepared to see already-type-checked nodes (see PR5755). In some
places, we had ugly workarounds to try to avoid re-type-checking
(e.g., in VarDecl initializer instantiation).

Now, we skip implicitly-generated nodes when performing instantiation,
preferring instead to build just the AST nodes that directly reflect
what was written in the source code. This has several advantages:

  - We don't need to instantiate anything that doesn't have a direct
    correlation to the source code, so we can have better location
    information.
  - Semantic analysis sees the same thing at template instantiation
    time that it would see for a non-template.
  - At least one ugly hack (VarDecl initializers) goes away.

Fixes PR5755.

llvm-svn: 91218