Allow the OSLog module to specify the log string section name

When building the `OSLog` module, look for a variable named
`osLogStringSectionName`. It must have a string literal as its
initializer, which provides the section name where the log strings
should be emitted. The `OSLog` module should contain something like
this:

    let osLogStringSectionName = "__TEXT,__logit"

When not present, the compiler will default to
`__TEXT,__oslogstring,cstring_literals`, which was previously the
hardcoded section name. Now, OSLog can customize the name.

Implements rdar://171571056

lib/Serialization/ModuleFileSharedCore.cpp

@@ -231,6 +231,9 @@ static bool readOptionsBlock(llvm::BitstreamCursor &cursor,
     case options_block::DEFERRED_CODE_GEN:
       extendedInfo.setDeferredCodeGen(true);
       break;
+    case options_block::OSLOG_STRING_SECTION_NAME:
+      extendedInfo.setOSLogStringSectionName(blobData);
+      break;
     default:
       // Unknown options record, possibly for use by a future version of the
       // module format.
@@ -1591,6 +1594,8 @@ ModuleFileSharedCore::ModuleFileSharedCore(
       ModulePackageName = extInfo.getModulePackageName();
       ModuleExportAsName = extInfo.getExportAsName();
       PublicModuleName = extInfo.getPublicModuleName();
+      OSLogStringSectionName = extInfo.getOSLogStringSectionName();
+      
       SwiftInterfaceCompilerVersion =
           extInfo.getSwiftInterfaceCompilerVersion();