Template instantiation for the declarations of member functions within

Template instantiation for the declarations of member functions within
a class template. At present, we can only instantiation normal
methods, but not constructors, destructors, or conversion operators.

As ever, this contains a bit of refactoring in Sema's type-checking. In
particular:

  - Split ActOnFunctionDeclarator into ActOnFunctionDeclarator
    (handling the declarator itself) and CheckFunctionDeclaration
    (checking for the the function declaration), the latter of which
    is also used by template instantiation.
  - We were performing the adjustment of function parameter types in
    three places; collect those into a single new routine.
  - When the type of a parameter is adjusted, allocate an
    OriginalParmVarDecl to keep track of the type as it was written.
  - Eliminate a redundant check for out-of-line declarations of member
    functions; hide more C++-specific checks on function declarations
    behind if(getLangOptions().CPlusPlus).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67575 91177308-0d34-0410-b5e6-96231b3b80d8