Solve compilation error on summit. (#12305)

* This constexpr does not compile on summit.

I tried gcc6.4 and gcc7.4

* Add link the the llvm bug report.

var/spack/repos/builtin/packages/llvm/constexpr_longdouble.patch

@@ -0,0 +1,15 @@
+--- a/projects/libcxx/include/thread
++++ b/projects/libcxx/include/thread
+@@ -435,7 +435,12 @@ sleep_for(const chrono::duration<_Rep, _Period>& __d)
+     using namespace chrono;
+     if (__d > duration<_Rep, _Period>::zero())
+     {
++#if ! (defined(_LIBCPP_COMPILER_GCC) && (__powerpc__ || __POWERPC__))
++    //  GCC's long double const folding is incomplete for IBM128 long doubles.
+         _LIBCPP_CONSTEXPR duration<long double> _Max = nanoseconds::max();
++#else
++        _LIBCPP_CONSTEXPR duration<long double> _Max = duration<long double>(ULLONG_MAX/1000000000ULL) ;
++#endif
+         nanoseconds __ns;
+         if (__d < _Max)
+         {

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

@@ -583,6 +583,10 @@ class Llvm(CMakePackage):
 
     # Github issue #4986
     patch('llvm_gcc7.patch', when='@4.0.0:4.0.1+lldb %gcc@7.0:')
+    # Backport from llvm master + additional fix
+    # see  https://bugs.llvm.org/show_bug.cgi?id=39696
+    # for a bug report about this problem in llvm master.
+    patch('constexpr_longdouble.patch', when='@7:8+libcxx')
 
     @run_before('cmake')
     def check_darwin_lldb_codesign_requirement(self):