[lldb][dotest] Improve libc++ detection

[rupprecht]

Summary

The test logic for running libc++ tests only looks to see if /usr/include/c++/v1 exists. This adds a fallback for including libc++ tests as long as $(CC) -stdlib=libc++ works.

[merge_guards_bot]

Build result: pass - 60689 tests passed, 0 failed and 726 were skipped.

Log files: console-log.txt, CMakeCache.txt

[labath]

This seems ok. The reason these checks are written this way is because we used to have the xcode build which bypassed cmake and lit, so this was the only place to implement that centrally. Nowadays, we could probably come up with a simpler way to do these things, but I don't have a clear idea on how to do that yet...

[rupprecht]

lldb/packages/Python/lldbsuite/test/dotest.py

859

FYI, the check didn't work because this isn't exercising anything in libc++, it really only checks that -stdlib=libc++ is a valid compiler flag. 786b6db8e6fd87fb82f2ad3e94e20c5c9cf9c4e4 fixes this by adding a libc++ include, which fails the command if libc++ is not installed where clang can find it.

lldb/packages/Python/lldbsuite/test/dotest.py

 import errno
 import logging
 import os
 import platform
 import re
 import signal
 import subprocess
 import sys
+import tempfile
 
 # Third-party modules
 import six
 import unittest2
 
 # LLDB Modules
 import lldbsuite
 from . import configuration
def canRunLibcxxTests():
     from lldbsuite.test import lldbplatformutil
 
     platform = lldbplatformutil.getPlatform()
 
     if lldbplatformutil.target_is_android() or lldbplatformutil.platformIsDarwin():
         return True, "libc++ always present"
 
     if platform == "linux":
-        if not os.path.isdir("/usr/include/c++/v1"):
-            return False, "Unable to find libc++ installation"
+        if os.path.isdir("/usr/include/c++/v1"):
             return True, "Headers found, let's hope they work"
+        with tempfile.NamedTemporaryFile() as f:
+            cmd = [configuration.compiler, "-xc++", "-stdlib=libc++", "-o", f.name, "-"]
+            p = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=True)
+            _, stderr = p.communicate("int main() {}")
+            if not p.returncode:
+                return True, "Compiling with -stdlib=libc++ works"
+            return False, "Compiling with -stdlib=libc++ fails with the error: %s" % stderr
 
     return False, "Don't know how to build with libc++ on %s" % platform
 
 def checkLibcxxSupport():
     result, reason = canRunLibcxxTests()
     if result:
         return # libc++ supported
     if "libc++" in configuration.categoriesList: