Have Sema::ActOnStartOfFunctionDef return the declaration that was passed it.

This fixes the missing warning here:

struct S {
    template <typename T>
    void meth() {
        char arr[3];
        arr[4] = 0; // warning: array index 4 is past the end of the array
    }
};

template <typename T>
void func() {
    char arr[3];
    arr[4] = 0; // no warning
}

llvm-svn: 170180