Add LLDB dataformatters for llvm::StringRef and lldb_private::ConstString

[aprantl]

Summary

These data formatters make the string value appear in Xcode's variables view (and on the command line) without having to expand the data structure.

[jingham]

I don't think you need to go back and cover all the ones already added, but could we start adding tests for new lldb/llvm data formatters as we add them? That way we can keep them useful as people change llvm.

[davide]

LGTM. It would be great if we can add a test.

[aprantl]

It's not quite obvious to me how to write a test for this. The test would need to live in LLDB, and it would need to compile a program linking against a variant of libSupport that was compiled with debug info. I'm not sure how to best do this.

llvm/utils/lldbDataFormatters.py

debugger.HandleCommand('type synthetic add -w llvm '
                            '-l lldbDataFormatters.ArrayRefSynthProvider '
                            '-x "^llvm::ArrayRef<.+>$"')
     debugger.HandleCommand('type summary add -w llvm '
                            '-F lldbDataFormatters.OptionalSummaryProvider '
                            '-x "^llvm::Optional<.+>$"')
     debugger.HandleCommand('type summary add -w llvm '
                            '-F lldbDataFormatters.SmallStringSummaryProvider '
                            '-x "^llvm::SmallString<.+>$"')
+    debugger.HandleCommand('type summary add -w llvm '
+                           '-F lldbDataFormatters.StringRefSummaryProvider '
+                           '-x "^llvm::StringRef$"')
+    debugger.HandleCommand('type summary add -w llvm '
+                           '-F lldbDataFormatters.ConstStringSummaryProvider '
+                           '-x "^lldb_private::ConstString$"')
 
 # Pretty printer for llvm::SmallVector/llvm::SmallVectorImpl
 class SmallVectorSynthProvider:
     def __init__(self, valobj, dict):
         self.valobj = valobj;
         self.update() # initialize this provider
 
     def num_children(self):

 
 def SmallStringSummaryProvider(valobj, internal_dict):
     num_elements = valobj.GetNumChildren()
     res = "\""
     for i in range(0, num_elements):
       res += valobj.GetChildAtIndex(i).GetValue().strip("'")
     res += "\""
     return res
+
+def StringRefSummaryProvider(valobj, internal_dict):
+    if valobj.GetNumChildren() == 2:
+        max_length = 256
+        length = valobj.GetChildAtIndex(1).GetValueAsUnsigned(max_length)
+        if length == 0:
+            return "NULL"
+        if length < max_length:
+            return valobj.GetChildAtIndex(0).GetSummary()
+    return ""
+
+def ConstStringSummaryProvider(valobj, internal_dict):
+    if valobj.GetNumChildren() == 1:
+        return valobj.GetChildAtIndex(0).GetSummary()
+    return ""