[Runtime] Remove dependency on Compiler.h from Config.h.

The runtime doesn't really need Compiler.h. It just needs some
visibility macros which can be inlined here instead of pulling
the whole heavyweight header (including its transitive closure,
llvm-config.h). This is becoming more important now that Compiler.h
includes C++ headers (namely, <new>), and swift/Runtime/Config.h
can be included from C or Objective-C files (causing build failures).

<rdar://problem/35860874>

Author: Davide Italiano

include/swift/Runtime/Config.h

@@ -17,8 +17,34 @@
 #ifndef SWIFT_RUNTIME_CONFIG_H
 #define SWIFT_RUNTIME_CONFIG_H
 
-// Bring in visibility attribute macros for library visibility.
-#include "llvm/Support/Compiler.h"
+/// \macro SWIFT_RUNTIME_GNUC_PREREQ
+/// Extend the default __GNUC_PREREQ even if glibc's features.h isn't
+/// available.
+#ifndef SWIFT_RUNTIME_GNUC_PREREQ
+# if defined(__GNUC__) && defined(__GNUC_MINOR__) && defined(__GNUC_PATCHLEVEL__)
+#  define SWIFT_RUNTIME_GNUC_PREREQ(maj, min, patch) \
+    ((__GNUC__ << 20) + (__GNUC_MINOR__ << 10) + __GNUC_PATCHLEVEL__ >= \
+     ((maj) << 20) + ((min) << 10) + (patch))
+# elif defined(__GNUC__) && defined(__GNUC_MINOR__)
+#  define SWIFT_RUNTIME_GNUC_PREREQ(maj, min, patch) \
+    ((__GNUC__ << 20) + (__GNUC_MINOR__ << 10) >= ((maj) << 20) + ((min) << 10))
+# else
+#  define SWIFT_RUNTIME_GNUC_PREREQ(maj, min, patch) 0
+# endif
+#endif
+
+/// SWIFT_RUNTIME_LIBRARY_VISIBILITY - If a class marked with this attribute is
+/// linked into a shared library, then the class should be private to the
+/// library and not accessible from outside it.  Can also be used to mark
+/// variables and functions, making them private to any shared library they are
+/// linked into.
+/// On PE/COFF targets, library visibility is the default, so this isn't needed.
+#if (__has_attribute(visibility) || SWIFT_RUNTIME_GNUC_PREREQ(4, 0, 0)) &&    \
+    !defined(__MINGW32__) && !defined(__CYGWIN__) && !defined(_WIN32)
+#define SWIFT_RUNTIME_LIBRARY_VISIBILITY __attribute__ ((visibility("hidden")))
+#else
+#define SWIFT_RUNTIME_LIBRARY_VISIBILITY
+#endif
 
 /// Does the current Swift platform support "unbridged" interoperation
 /// with Objective-C?  If so, the implementations of various types must

stdlib/private/StdlibUnittest/GetOSVersion.mm

@@ -15,7 +15,7 @@
 
 #include "swift/Runtime/Config.h"
 
-SWIFT_CC(swift) LLVM_LIBRARY_VISIBILITY extern "C"
+SWIFT_CC(swift) SWIFT_RUNTIME_LIBRARY_VISIBILITY extern "C"
 const char *
 getSystemVersionPlistProperty(const char *PropertyName) {
   // This function is implemented in Objective-C because Swift does not support

stdlib/private/StdlibUnittest/InterceptTraps.cpp

@@ -33,7 +33,7 @@ static void CrashCatcher(int Sig) {
   _exit(0);
 }
 
-SWIFT_CC(swift) LLVM_LIBRARY_VISIBILITY extern "C"
+SWIFT_CC(swift) SWIFT_RUNTIME_LIBRARY_VISIBILITY extern "C"
 void installTrapInterceptor() {
   // Disable buffering on stdout so that everything is printed before crashing.
   setbuf(stdout, 0);

stdlib/private/StdlibUnittest/OpaqueIdentityFunctions.cpp

@@ -12,6 +12,6 @@
 
 #include "swift/Runtime/Config.h"
 
-SWIFT_CC(swift) LLVM_LIBRARY_VISIBILITY extern "C"
+SWIFT_CC(swift) SWIFT_RUNTIME_LIBRARY_VISIBILITY extern "C"
 void *getPointer(void *x) { return x; }
 

stdlib/private/StdlibUnittestFoundationExtras/UnavailableFoundationMethodThunks.mm

@@ -13,7 +13,7 @@
 #include <Foundation/Foundation.h>
 #include "swift/Runtime/Config.h"
 
-SWIFT_CC(swift) LLVM_LIBRARY_VISIBILITY
+SWIFT_CC(swift) SWIFT_RUNTIME_LIBRARY_VISIBILITY
 extern "C" void
 NSArray_getObjects(NSArray SWIFT_NS_RELEASES_ARGUMENT *_Nonnull nsArray,
                    id *objects, NSUInteger rangeLocation,
@@ -22,7 +22,7 @@
   SWIFT_CC_PLUSONE_GUARD([nsArray release]);
 }
 
-SWIFT_CC(swift) LLVM_LIBRARY_VISIBILITY
+SWIFT_CC(swift) SWIFT_RUNTIME_LIBRARY_VISIBILITY
 extern "C" void
 NSDictionary_getObjects(NSDictionary *_Nonnull nsDictionary,
                         id *objects, id *keys) {