SILGen: Avoid expanding #line directives with invalid source locations.

Synthesized stub constructors contain `#line` directives and under certain
circumstances these directives can have invalid source locations. SILGen would
trigger an assert (or invoke UB in non-asserts builds) when attempting to
expand these directives into literal values. Check the source location for
validity before translating the location to a location in the outermost source
file.

Resolves rdar://121971741

lib/SILGen/SILGenApply.cpp

@@ -2179,14 +2179,15 @@ buildBuiltinLiteralArgs(SILGenFunction &SGF, SGFContext C,
 
   case MagicIdentifierLiteralExpr::Line:
   case MagicIdentifierLiteralExpr::Column: {
-    auto Loc = getLocInOutermostSourceFile(SGF.getSourceManager(),
-                                           magicLiteral->getStartLoc());
     unsigned Value = 0;
+    if (auto Loc = magicLiteral->getStartLoc()) {
+      Loc = getLocInOutermostSourceFile(SGF.getSourceManager(), Loc);
       if (Loc.isValid()) {
         Value = magicLiteral->getKind() == MagicIdentifierLiteralExpr::Line
                     ? ctx.SourceMgr.getPresumedLineAndColumnForLoc(Loc).first
                     : ctx.SourceMgr.getPresumedLineAndColumnForLoc(Loc).second;
       }
+    }
 
     auto silTy = SILType::getBuiltinIntegerLiteralType(ctx);
     auto ty = silTy.getASTType();

test/SILGen/Inputs/objc_implementation.h

@@ -0,0 +1,12 @@
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface ImplClass : NSObject
+
+- (instancetype)init NS_UNAVAILABLE;
+- (instancetype)initWithInt:(NSInteger)x;
+
+@end
+
+NS_ASSUME_NONNULL_END

test/SILGen/objc_implementation.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -emit-silgen -import-objc-header %S/Inputs/objc_implementation.h -swift-version 5 %s | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+@_objcImplementation extension ImplClass {
+  public init(int: Int) {
+    super.init()
+  }
+
+  // CHECK-LABEL: sil{{.*}} @$sSo9ImplClassC19objc_implementationEABycfc : $@convention(method) (@owned ImplClass) -> @owned ImplClass {
+  // CHECK:         function_ref @$ss25_unimplementedInitializer9className04initD04file4line6columns5NeverOs12StaticStringV_A2JS2utF
+  // CHECK:       } // end sil function '$sSo9ImplClassC19objc_implementationEABycfc'
+}