[Sema] Split availability processing into SemaAvailability.cpp

Reduces compile time of SemaDeclAttr.cpp down to 28s from 50s. The new
TU does a few RecursiveASTVisitor instantiations, so it takes 30s.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D73385

Author: rnk

clang/include/clang/Sema/Sema.h

@@ -4424,6 +4424,8 @@ class Sema final {
   /// Issue any -Wunguarded-availability warnings in \c FD
   void DiagnoseUnguardedAvailabilityViolations(Decl *FD);
 
+  void handleDelayedAvailabilityCheck(sema::DelayedDiagnostic &DD, Decl *Ctx);
+
   //===--------------------------------------------------------------------===//
   // Expression Parsing Callbacks: SemaExpr.cpp.
 

clang/lib/Sema/CMakeLists.txt

@@ -29,6 +29,7 @@ add_clang_library(clangSema
   Sema.cpp
   SemaAccess.cpp
   SemaAttr.cpp
+  SemaAvailability.cpp
   SemaCXXScopeSpec.cpp
   SemaCast.cpp
   SemaChecking.cpp