(possible implicit) noexcept specifier

Throwing in the destructor is not good (C++11 change try to not allow see below).
 But in reality, those codes are exist.
C++11 [class.dtor]p3:

A declaration of a destructor that does not have an exception-specification is 
implicitly considered to have the same exception specification as an implicit 
declaration.

With this change, the application worked before may now run into runtime 
termination. My goal here is to emit a warning to provide only possible info to 
where the code may need to be changed.

First there is no way, in compile time to identify the “throw” really throw out 
of the function. Things like the call which throw out… To keep this simple, 
when “throw” is seen, checking its enclosing function(only destructor and 
dealloc functions) with noexcept(true) specifier emit warning.

Here is implementation detail:
A new member function CheckCXXThrowInNonThrowingFunc is added for class Sema 
in Sema.h. It is used in the call to both BuildCXXThrow and 
TransformCXXThrowExpr.

The function basic check if the enclosing function with non-throwing noexcept 
specifer, if so emit warning for it.

The example of warning message like:
k1.cpp:18:3: warning: ''~dependent_warn'' has a (possible implicit) non-throwing

    noexcept specifier. Throwing exception may cause termination.
        [-Wthrow-in-dtor]
        throw 1;
        ^

        k1.cpp:43:30: note: in instantiation of member function

        'dependent_warn<noexcept_fun>::~dependent_warn' requested here

        dependent_warn<noexcept_fun> f; // cause warning

Differential Revision: https://reviews.llvm.org/D33333



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@306149 91177308-0d34-0410-b5e6-96231b3b80d8