Remove hard-coded standard C++ library selection and add more releases in llvm package (#19933)

* Restore OS based Clang default choice of C++ standard library.

* Add LLVM 11.0.1 release

var/spack/repos/builtin/packages/llvm/package.py

@@ -28,11 +28,13 @@ class Llvm(CMakePackage, CudaPackage):
 
     # fmt: off
     version('master', branch='master')
+    version('11.0.1', sha256='9c7ad8e8ec77c5bde8eb4afa105a318fd1ded7dff3747d14f012758719d7171b')
     version('11.0.0', sha256='8ad4ddbafac4f2c8f2ea523c2c4196f940e8e16f9e635210537582a48622a5d5')
     version('10.0.1', sha256='c7ccb735c37b4ec470f66a6c35fbae4f029c0f88038f6977180b1a8ddc255637')
     version('10.0.0', sha256='b81c96d2f8f40dc61b14a167513d87c0d813aae0251e06e11ae8a4384ca15451')
     version('9.0.1', sha256='be7b034641a5fda51ffca7f5d840b1a768737779f75f7c4fd18fe2d37820289a')
     version('9.0.0', sha256='7807fac25330e24e9955ca46cd855dd34bbc9cc4fdba8322366206654d1036f2')
+    version('8.0.1', sha256='5b18f6111c7aee7c0933c355877d4abcfe6cb40c1a64178f28821849c725c841')
     version('8.0.0', sha256='d81238b4a69e93e29f74ce56f8107cbfcf0c7d7b40510b7879e98cc031e25167')
     version('7.1.0', sha256='71c93979f20e01f1a1cc839a247945f556fa5e63abf2084e8468b238080fd839')
     version('7.0.1', sha256='f17a6cd401e8fd8f811fbfbb36dcb4f455f898c9d03af4044807ad005df9f3c0')
@@ -443,8 +445,6 @@ def cmake_args(self):
         if "+libcxx" in spec:
             projects.append("libcxx")
             projects.append("libcxxabi")
-            if spec.satisfies("@3.9.0:"):
-                cmake_args.append("-DCLANG_DEFAULT_CXX_STDLIB=libc++")
         if "+mlir" in spec:
             projects.append("mlir")
         if "+internal_unwind" in spec: