[lldb] Let dotest check for the `cassert` header instead of `algorithm` for libc++ check

[teemperor]

Summary

This is just a dotest check to see if we can compile a simple program that uses libc++. Right now we are
parsing the rather big algorithm header in the test program, but the test really just checks
whether we can find *any* libc++ headers and link against some libc++ SO. Using the much smaller
cassert header for checking whether we can find libc++ headers speeds up this check by a bit.

After some incredibly unscientific performance testing this saves a few seconds when running the
test suite on Linux (on macOS we hardcoded that libc++ is always there, so this check won't be
used there and we don't save any time).

[jankratochvil]

The detection still works fine with libcxx-devel-11.0.0-1.fc33.x86_64.

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

def canRunLibcxxTests():
 
     if lldbplatformutil.target_is_android() or lldbplatformutil.platformIsDarwin():
         return True, "libc++ always present"
 
     if platform == "linux":
         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("#include <algorithm>\nint main() {}")
+            _, stderr = p.communicate("#include <cassert>\nint 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()