Replace the workaround from r153445 with a proper fix.

Infinite recursion was happening when DiagnoseInvalidRedeclaration
called ActOnFunctionDeclarator to check if a typo correction works when
the correction was just to the nested-name-specifier because the wrong
DeclContext was being passed in. Unlike a number of functions
surrounding typo correction, the DeclContext passed in for a function is
the context of the function name after applying any nested name
specifiers, not the lexical DeclContext where the
function+nested-name-specifier appears.

llvm-svn: 153962