standard library: remove use of LLVM.h in the compatibility

Remove the use of the `LLVM.h` forward declarations from the
compatibility shims.  This allows us to fully isolate the target side
from the LLVM namespace ensuring that we prevent ODR violations when
LLVM is linked into the same address space.

Author: compnerd

stdlib/toolchain/Compatibility50/Overrides.h

@@ -14,7 +14,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "swift/Basic/LLVM.h"
 #include "swift/Runtime/Metadata.h"
 
 namespace swift {

stdlib/toolchain/Compatibility51/Overrides.h

@@ -14,20 +14,20 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "swift/Basic/LLVM.h"
 #include "swift/Runtime/Metadata.h"
+#include "llvm/ADT/StringRef.h"
 
 namespace swift {
 
 using ConformsToSwiftProtocol_t =
   const ProtocolConformanceDescriptor *(const Metadata * const type,
                                         const ProtocolDescriptor *protocol,
-                                        StringRef moduleName);
+                                        llvm::StringRef moduleName);
 
 const ProtocolConformanceDescriptor *
 swift51override_conformsToSwiftProtocol(const Metadata * const type,
                                         const ProtocolDescriptor *protocol,
-                                        StringRef moduleName,
+                                        llvm::StringRef moduleName,
                                         ConformsToSwiftProtocol_t *original);
 
 } // end namespace swift