Add Attribute NoThrow as an Exception Specifier Type

In response to https://bugs.llvm.org/show_bug.cgi?id=33235, it became
clear that the current mechanism of hacking through checks for the
exception specification of a function gets confused really quickly when
there are alternate exception specifiers.

This patch introcues EST_NoThrow, which is the equivilent of
EST_noexcept when caused by EST_noThrow. The existing implementation is
left in place to cover functions with no FunctionProtoType.

Differential Revision: https://reviews.llvm.org/D62435

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@362119 91177308-0d34-0410-b5e6-96231b3b80d8