Start handling variable declarations (#571)

TODOs in the test for known issues.

I may switch the approach to getting the variable type (on examination, this isn't working quite as well as I'd thought) but for now I think it's okay. I had an earlier approach though that may work better overall -- I'd been thinking this would work better, but as you can see in the null check for type information, I think I missed a key point.

Anyways, what'd really been vexing me was `int i, j` which I think I handle passably well now. There's obviously room for improvement, but given I've been going at this for a couple days now, I thought it best to checkpoint where I was.

This also includes some related framework changes to fix bumps I was running into. Overall the tool should operate a bit more smoothly with these changes. There are still issues with overlapping replacements, but I think it's primarily with range-based for loops which I just need to take some time to fix.

migrate_cpp/cpp_refactoring/matcher.cpp

@@ -12,19 +12,16 @@ void Matcher::AddReplacement(const clang::SourceManager& sm,
                              clang::CharSourceRange range,
                              llvm::StringRef replacement_text) {
   if (!range.isValid()) {
-    llvm::errs() << "Invalid range: " << range.getAsRange().printToString(sm)
-                 << "\n";
+    // Invalid range.
     return;
   }
   if (sm.getDecomposedLoc(range.getBegin()).first !=
       sm.getDecomposedLoc(range.getEnd()).first) {
-    llvm::errs() << "Range spans macro expansions: "
-                 << range.getAsRange().printToString(sm) << "\n";
+    // Range spans macro expansions.
     return;
   }
   if (sm.getFileID(range.getBegin()) != sm.getFileID(range.getEnd())) {
-    llvm::errs() << "Range spans files: "
-                 << range.getAsRange().printToString(sm) << "\n";
+    // Range spans files.
     return;
   }
 
@@ -38,8 +35,8 @@ void Matcher::AddReplacement(const clang::SourceManager& sm,
 
   auto err = entry->second.add(rep);
   if (err) {
-    llvm::report_fatal_error("Error with replacement `" + rep.toString() +
-                             "`: " + llvm::toString(std::move(err)) + "\n");
+    llvm::errs() << "Error with replacement `" << rep.toString()
+                 << "`: " << llvm::toString(std::move(err)) << "\n";
   }
 }