Make GetValueForVariableExpression use StringRef.

Also significantly reduced the indentation level by use of
early returns, and simplified some of the logic by using
StringRef functions such as consumeInteger() and getAsInteger()
instead of strtoll, etc.

llvm-svn: 287189

lldb/include/lldb/Target/StackFrame.h

@@ -313,7 +313,7 @@ public:
   ///     A shared pointer to the ValueObject described by var_expr.
   //------------------------------------------------------------------
   lldb::ValueObjectSP GetValueForVariableExpressionPath(
-      const char *var_expr, lldb::DynamicValueType use_dynamic,
+      llvm::StringRef var_expr, lldb::DynamicValueType use_dynamic,
       uint32_t options, lldb::VariableSP &var_sp, Error &error);
 
   //------------------------------------------------------------------