[lldb][NFCI] Pre-allocate storage in ObjCLanguage::MethodName::GetFullNameWithoutCategory

The size of a full ObjC MethodName can vary somewhat, but it is
computable ahead of time.

Using a reasonably sized ObjC application, this actually improves the
time it takes to initialize symbol indexes for ObjC names ever so
slightly. Additionally, I found that the variability in time also was
improved considerably.

Differential Revision: https://reviews.llvm.org/D150914

lldb/source/Plugins/Language/ObjC/ObjCLanguage.cpp

@@ -152,7 +152,15 @@ std::string ObjCLanguage::MethodName::GetFullNameWithoutCategory() const {
 
   llvm::StringRef class_name = GetClassName();
   llvm::StringRef selector_name = GetSelector();
+
+  // Compute the total size to avoid reallocations
+  // class name + selector name + '[' + ' ' + ']'
+  size_t total_size = class_name.size() + selector_name.size() + 3;
+  if (m_type != eTypeUnspecified)
+    total_size++; // For + or -
+
   std::string name_sans_category;
+  name_sans_category.reserve(total_size);
 
   if (m_type == eTypeClassMethod)
     name_sans_category += '+';