Function definition may have uninstantiated body

Current implementation of `FunctionDecl::isDefined` does not take into
account redeclarations that do not have bodies, but the bodies can be
instantiated from corresponding templated definition. This behavior does
not allow to detect function redefinition in the cases where friend
functions is defined in class templates. For instance, the code:
```
    template<typename T> struct X { friend void f() {} };
    X<int> xi;
    void f() {}
```
compiles successfully but must fail due to redefinition of `f`. The
declaration of the friend `f` is created when the containing template
`X` is instantiated, but it does not have a body as per 14.5.4p4
because `f` is not odr-used.

With this change the function `Sema::CheckForFunctionRedefinition`
considers functions with uninstantiated bodies as definitions.

Differential Revision: https://reviews.llvm.org/D30170

llvm-svn: 326419