Changes

[libc][NFC] Use ASSERT_FP_EQ to comapre NaN values in tests.

This is a continuation of the previous CL which did a similar change in
other tests. To elaborate a little about why we need this - under C++
compilation with headers not from LLVM libc, libraries like libc++ and
libstdc++ provide their own math.h which undefine macros like `isnan`
and provide the overloaded C++ isnan functions which return a boolean
value instead of an integer value returned by the isnan macro.

DR2064: decltype(E) is only a dependent type if E is type-dependent, not
if E is merely instantiation-dependent.

Where possible, don't try to ask whether a template argument is
dependent until it's been converted to match its parameter.

The type of a non-type template parameter can in general affect whether
the template argument is dependent.

Note that this is not always possible. For template arguments that name
static local variables in templates, the type of the template parameter
affects whether the argument is dependent, so the query is imprecise
until we know the parameter type. For example, in:

template<typename T> void f() {
  static const int n = 5;
  typename T::template X<n> x;
}

... we don't know whether 'n' is dependent until we know whether the
corresponding template parameter is of type 'int' or 'const int&'.

Ensure that we transform types into the current instantiation even if
they're only instantiation-dependent.