[Sema] Intro flag to limit availability checks to the API

Intro the frontend flag `-check-api-availability-only` that limits
availability checking to the API and SPI. This mode doesn't check the
availability of non-inlinable function bodies and of non-ABI-public decl
signatures.

This mode goal is to check all that is printed in the swiftinterface
file. It should be used in place of the wider
`-disable-availability-checking` when generating an interface for
platforms with no binaries.

rdar://81679692

lib/Frontend/CompilerInvocation.cpp

@@ -473,6 +473,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.DisableAvailabilityChecking |=
       Args.hasArg(OPT_disable_availability_checking);
+  Opts.CheckAPIAvailabilityOnly |=
+      Args.hasArg(OPT_check_api_availability_only);
 
   if (auto A = Args.getLastArg(OPT_enable_conformance_availability_errors,
                                OPT_disable_conformance_availability_errors)) {
@@ -915,6 +917,12 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
   Opts.DebugTimeExpressions |=
       Args.hasArg(OPT_debug_time_expression_type_checking);
 
+  // Checking availability of the API only relies on skipping non-inlinable
+  // function bodies. Define it first so it can be overridden by the other
+  // flags.
+  if (Args.hasArg(OPT_check_api_availability_only))
+    Opts.SkipFunctionBodies = FunctionBodySkipping::NonInlinable;
+
   // Check for SkipFunctionBodies arguments in order from skipping less to
   // skipping more.
   if (Args.hasArg(