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/BUILD

@@ -11,28 +11,11 @@ cc_binary(
     srcs = ["main.cpp"],
     deps = [
         ":fn_inserter",
+        ":var_decl",
         "@llvm-project//clang:tooling",
     ],
 )
 
-cc_library(
-    name = "fn_inserter",
-    srcs = ["fn_inserter.cpp"],
-    hdrs = ["fn_inserter.h"],
-    deps = [":matcher"],
-)
-
-cc_test(
-    name = "fn_inserter_test",
-    srcs = ["fn_inserter_test.cpp"],
-    deps = [
-        ":fn_inserter",
-        ":matcher_test_base",
-        "@llvm-project//clang:tooling",
-        "@llvm-project//llvm:gtest_main",
-    ],
-)
-
 cc_library(
     name = "matcher",
     srcs = ["matcher.cpp"],
@@ -55,3 +38,41 @@ cc_library(
         "@llvm-project//llvm:gtest",
     ],
 )
+
+# Individual matchers
+
+cc_library(
+    name = "fn_inserter",
+    srcs = ["fn_inserter.cpp"],
+    hdrs = ["fn_inserter.h"],
+    deps = [":matcher"],
+)
+
+cc_test(
+    name = "fn_inserter_test",
+    srcs = ["fn_inserter_test.cpp"],
+    deps = [
+        ":fn_inserter",
+        ":matcher_test_base",
+        "@llvm-project//clang:tooling",
+        "@llvm-project//llvm:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "var_decl",
+    srcs = ["var_decl.cpp"],
+    hdrs = ["var_decl.h"],
+    deps = [":matcher"],
+)
+
+cc_test(
+    name = "var_decl_test",
+    srcs = ["var_decl_test.cpp"],
+    deps = [
+        ":matcher_test_base",
+        ":var_decl",
+        "@llvm-project//clang:tooling",
+        "@llvm-project//llvm:gtest_main",
+    ],
+)