When a function template's template parameter has a default argument,

When a function template's template parameter has a default argument,
it's okay for the following template parameters to not have default
arguments (since those template parameters can still be
deduced). Also, downgrade the error about default template arguments
in function templates to an extension warning, since this is a
harmless C++0x extension.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124855 91177308-0d34-0410-b5e6-96231b3b80d8