Set default cmake build_type to Release for llvm

Override CMake "build_type" variant to default to "Release" for
llvm package.

lib/spack/spack/build_systems/cmake.py

@@ -84,7 +84,7 @@ class CMakePackage(PackageBase):
 
     # https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html
     variant('build_type', default='RelWithDebInfo',
-            description='The build type to build',
+            description='CMake build type',
             values=('Debug', 'Release', 'RelWithDebInfo', 'MinSizeRel'))
 
     depends_on('cmake', type='build')

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

@@ -70,6 +70,9 @@ class Llvm(CMakePackage):
     variant('all_targets', default=True,
             description="Build all supported targets, default targets "
             "<current arch>,NVPTX,AMDGPU,CppBackend")
+    variant('build_type', default='Release',
+            description='CMake build type',
+            values=('Debug', 'Release', 'RelWithDebInfo', 'MinSizeRel'))
 
     # Build dependency
     depends_on('cmake@3.4.3:', type='build')