Fix treatment of types defined in function prototype

Types defined in function prototype are diagnosed earlier in C++ compilation.
They are put into declaration context where the prototype is introduced. Later on,
when FunctionDecl object is created, these types are moved into the function context.

This patch fixes PR19018 and PR18963.

Differential Revision: http://reviews.llvm.org/D4145


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211718 91177308-0d34-0410-b5e6-96231b3b80d8