Correct misspelling of "availability" in a few places.

Author: tshortli

include/swift/AST/ASTContext.h

@@ -909,7 +909,7 @@ class ASTContext final {
   AvailabilityRange getSwiftAvailability(unsigned major, unsigned minor) const;
 
   // For each feature defined in FeatureAvailability, define two functions;
-  // the latter, with the suffix RuntimeAvailabilty, is for use with
+  // the latter, with the suffix RuntimeAvailability, is for use with
   // AvailabilityRange::forRuntimeTarget(), and only looks at the Swift
   // runtime version.
 #define FEATURE(N, V)                                                          \

lib/AST/AvailabilityScopeBuilder.cpp

@@ -126,7 +126,7 @@ class AvailabilityScopeBuilder : private ASTWalker {
   }
 
   const char *stackTraceAction() const {
-    return "building availabilty scope for";
+    return "building availability scope for";
   }
 
   friend class swift::ExpandChildAvailabilityScopesRequest;
@@ -415,7 +415,7 @@ class AvailabilityScopeBuilder : private ASTWalker {
     // As a special case, extension decls are treated as effectively as
     // available as the nominal type they extend, up to the deployment target.
     // This rule is a convenience for library authors who have written
-    // extensions without specifying platform availabilty on the extension
+    // extensions without specifying platform availability on the extension
     // itself.
     if (auto *extension = dyn_cast<ExtensionDecl>(decl)) {
       auto extendedType = extension->getExtendedType();

lib/ClangImporter/ImportDecl.cpp

@@ -9746,7 +9746,7 @@ void ClangImporter::Implementation::importAttributes(
       declContext->lookupAvailabilityDomains(domainIdentifier, results);
 
       if (results.size() > 0) {
-        // FIXME: [availability] Diagnose ambiguous availabilty domain name?
+        // FIXME: [availability] Diagnose ambiguous availability domain name?
         auto AttrKind = avail->getUnavailable()
                             ? AvailableAttr::Kind::Unavailable
                             : AvailableAttr::Kind::Default;

lib/SILGen/SILGenAvailability.cpp

@@ -267,7 +267,7 @@ SILGenFunction::emitIfAvailableQuery(SILLocation loc,
 
   // The query may not have been computed by Sema under the following
   // conditions:
-  // - Availability checking was disabled (-disable-availabilty-checking).
+  // - Availability checking was disabled (-disable-availability-checking).
   // - The query was marked invalid in the AST for a non-fatal reason.
   //
   // Otherwise, there's a bug in Sema.