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

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).

Reviewed By: jankratochvil

Differential Revision: https://reviews.llvm.org/D101056

Author: Teemperor

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

@@ -751,7 +751,7 @@ def canRunLibcxxTests():
         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