AST: Implement mangling support for function types without a prototype.

Function types without prototypes can arise when mangling a function type
within an overloadable function in C. We mangle these as the absence of
any parameter types (not even an empty parameter list).

Differential Revision: http://reviews.llvm.org/D11848

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244374 91177308-0d34-0410-b5e6-96231b3b80d8