Make lldb::Regex use StringRef.

This updates getters and setters to use StringRef instead of
const char *.  I tested the build on Linux, Windows, and OSX
and saw no build or test failures.  I cannot test any BSD
or Android variants, however I expect the required changes
to be minimal or non-existant.

llvm-svn: 282079
diff --git a/lldb/source/Commands/CommandCompletions.cpp b/lldb/source/Commands/CommandCompletions.cpp
index 9d70f40..5eebc78 100644
--- a/lldb/source/Commands/CommandCompletions.cpp
+++ b/lldb/source/Commands/CommandCompletions.cpp
@@ -511,7 +511,7 @@
     pos = regex_str.insert(pos, '\\');
     pos = find_if(pos + 2, regex_str.end(), regex_chars);
   }
-  m_regex.Compile(regex_str.c_str());
+  m_regex.Compile(regex_str);
 }
 
 Searcher::Depth CommandCompletions::SymbolCompleter::GetDepth() {