Add universal build_type variant to CMakePackage (#4797)

* Add universal build_type variant to CMakePackage
* Override build_type in some packages with different possible values
* Remove reference to no longer existent debug variant
* Update CBTF packages with new build_type variant
* Keep note on build size of LLVM

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

@@ -43,10 +43,10 @@ class Llvm(CMakePackage):
     version('3.0', 'a8e5f5f1c1adebae7b4a654c376a6005',
             url='http://llvm.org/releases/3.0/llvm-3.0.tar.gz')
 
-    variant('debug', default=False,
-            description="Build a debug version of LLVM, this increases "
-            "binary size by an order of magnitude, make sure you have "
-            "20-30gb of space available to build this")
+    # NOTE: The debug version of LLVM is an order of magnitude larger than
+    # the release version, and may take up 20-30 GB of space. If you want
+    # to save space, build with `build_type=Release`.
+
     variant('clang', default=True,
             description="Build the LLVM C/C++/Objective-C compiler frontend")
     variant('lldb', default=True, description="Build the LLVM debugger")
@@ -327,12 +327,6 @@ class Llvm(CMakePackage):
     def setup_environment(self, spack_env, run_env):
         spack_env.append_flags('CXXFLAGS', self.compiler.cxx11_flag)
 
-    def build_type(self):
-        if '+debug' in self.spec:
-            return 'RelWithDebInfo'
-        else:
-            return 'Release'
-
     def cmake_args(self):
         spec = self.spec