llvm: fix PythonString::GetString for >=python-3.7 (#13631)

* llvm: fix PythonString::GetString for >=python-3.7

* llvm/package.py: fix when the patch should be applied

var/spack/repos/builtin/packages/llvm/llvm_py37.patch

@@ -0,0 +1,37 @@
+From ecdefed7f6ba11421fe1ecc6c13a135ab7bcda73 Mon Sep 17 00:00:00 2001
+From: Pavel Labath <labath@google.com>
+Date: Mon, 23 Jul 2018 11:37:36 +0100
+Subject: [PATCH] Fix PythonString::GetString for >=python-3.7
+
+The return value of PyUnicode_AsUTF8AndSize is now "const char *".
+---
+ .../Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp  | 6 ++++--
+ 1 file changed, 4 insertions(+), 2 deletions(-)
+
+diff --git a/tools/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp b/tools/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp
+index 6a9d57d5a..94f16b2c7 100644
+--- a/tools/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp
++++ b/tools/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp
+@@ -404,14 +404,16 @@ llvm::StringRef PythonString::GetString() const {
+     return llvm::StringRef();
+ 
+   Py_ssize_t size;
+-  char *c;
++  const char *data;
+ 
+ #if PY_MAJOR_VERSION >= 3
+-  c = PyUnicode_AsUTF8AndSize(m_py_obj, &size);
++  data = PyUnicode_AsUTF8AndSize(m_py_obj, &size);
+ #else
++  char *c;
+   PyString_AsStringAndSize(m_py_obj, &c, &size);
++  data = c;
+ #endif
+-  return llvm::StringRef(c, size);
++  return llvm::StringRef(data, size);
+ }
+ 
+ size_t PythonString::GetSize() const {
+-- 
+2.18.0.233.g985f88cf7e-goog
+

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

@@ -604,6 +604,11 @@ class Llvm(CMakePackage):
     # for a bug report about this problem in llvm master.
     patch('constexpr_longdouble.patch', when='@6:8+libcxx')
 
+    # Backport from llvm master; see
+    # https://bugs.llvm.org/show_bug.cgi?id=38233
+    # for a bug report about this problem in llvm master.
+    patch('llvm_py37.patch', when='@4:6 ^python@3.7:')
+
     @run_before('cmake')
     def check_darwin_lldb_codesign_requirement(self):
         if not self.spec.satisfies('+lldb platform=darwin'):