Avoid using NoThrow Exception Specifier in non-C++ Modes.

As reported in https://bugs.llvm.org/show_bug.cgi?id=42113, there are a
number of locations in Clang where it is assumed that exception
specifications are only valid in C++ mode. Since the original
justification for the NoThrow Exception Specifier Type was C++ related,
this patch just makes C mode use the attribute-based nothrow handling.

Additionally, I noticed that the handling of non-prototype functions
regressed the behavior of the nothrow attribute, in part because it is
was listed in the function type macro(which I did in the previous
patch).  In reality, it should only be doing so in a conditional nature,
so this patch removes it there and puts it directly in the switch to be
handled correctly.

llvm-svn: 362607