[ASTImporter] Unify redecl chain tests as type parameterized tests

Summary:
This patch unifies all those tests which check the correctness of the
redecl chains. Previously we had several structurally very similar test
cases for each language construct (class, function, variable, function
template, ...).

We still use value-parameterized tests for the different AST
compatibility switches (-fdelayed-template-parsing, -fms-compatibility).
Gtest makes it possible to have either value-parameterized or
type-parameterized fixtures. However, we cannot have both value- and
type-parameterized test fixtures. So we use a value-parameterized test
fixture in the gtest sense. We intend to mimic gtest's type-parameters
via the type template parameter. We manually instantiate the different
tests with the each types.

After this patch I am planning to put the "generic redecl chain" related
tests into their own separate test file (in another patch).

Reviewers: a_sidorin, shafik, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Differential Revision: https://reviews.llvm.org/D57236

llvm-svn: 354259