[clang-tidy] Separate the check-facing interface

Summary:
Move ClangTidyCheck to a separate header/.cpp
Switch checks to #include "ClangTidyCheck.h"
Mention ClangTidyCheck.h in the docs

Reviewers: hokein, gribozavr, aaron.ballman

Reviewed By: hokein

Subscribers: mgorny, javed.absar, xazax.hun, arphaman, jdoerfert, llvm-commits, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D59714

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356890 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/gn/secondary/clang-tools-extra/clang-tidy/BUILD.gn b/utils/gn/secondary/clang-tools-extra/clang-tidy/BUILD.gn
index 2b0bb2b..415e0fc 100644
--- a/utils/gn/secondary/clang-tools-extra/clang-tidy/BUILD.gn
+++ b/utils/gn/secondary/clang-tools-extra/clang-tidy/BUILD.gn
@@ -28,6 +28,7 @@
 
   sources = [
     "ClangTidy.cpp",
+    "ClangTidyCheck.cpp",
     "ClangTidyDiagnosticConsumer.cpp",
     "ClangTidyModule.cpp",
     "ClangTidyOptions.cpp",