[libcxx] [test] Check for C++ headers before building a test that uses them

[mstorsjo]

Summary

When the libcxx test framework is executed within libunwind, there
are no standard C++ headers available (libunwind builds with
-nostdinc++, but doesn't add any libcxx headers to the include path).

Check that a test that includes <iostream> can be compiled before trying
to build and execute a test program that includes it.

Previously, the compile error here would block all libunwind tests from
executing altogether.

libcxx/utils/libcxx/test/features.py

for locale, alts in locales.items():
   DEFAULT_FEATURES.append(Feature(name='locale.{}'.format(locale),
                                   when=lambda cfg, alts=alts: hasAnyLocale(cfg, alts)))
 
 
 # Add features representing the target platform name: darwin, linux, windows, etc...
 DEFAULT_FEATURES += [
   Feature(name='darwin', when=lambda cfg: '__APPLE__' in compilerMacros(cfg)),
   Feature(name='windows', when=lambda cfg: '_WIN32' in compilerMacros(cfg)),
-  Feature(name='windows-dll', when=lambda cfg: '_WIN32' in compilerMacros(cfg) and programSucceeds(cfg, """
+  Feature(name='windows-dll', when=lambda cfg: '_WIN32' in compilerMacros(cfg) and sourceBuilds(cfg, """
+            #include <iostream>
+            int main(int, char**) { return 0; }
+          """) and programSucceeds(cfg, """
             #include <iostream>
             #include <windows.h>
             #include <winnt.h>
             int main(int, char**) {
               // Get a pointer to a data member that gets linked from the C++
               // library. This must be a data member (functions can get
               // thunk inside the calling executable), and must not be
               // something that is defined inline in headers.