Implement C++ DR481, which clarifies that the scope of template

Implement C++ DR481, which clarifies that the scope of template
parameters starts at the end of the template-parameter rather than at
the point where the template parameter name is encounted. For example,
given:

  typedef unsigned char T;
  template<typename T = T> struct X0 { };

The "T" in the default argument refers to the typedef of "unsigned
char", rather than referring to the newly-introduced template type
parameter 'T'.

Addresses <rdar://problem/8122812>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107354 91177308-0d34-0410-b5e6-96231b3b80d8