[Modules] Early-exit if ReadOptionsBlock fails to avoid crashing

If we didn't tell ReadOptionsBlock to allow failures then we can't
assume that the stream is not in the middle of a block if it returns
out-of-date. This was causing a crash when we tried to continue reading.

Also, it's just generally a good idea to early-exit if we're doing
implicit module builds, since we will want to immediately rebuild this
module anyway and there's no reason to waste time continuing after
failure.

rdar://problem/24114938

llvm-svn: 260563

clang/lib/Serialization/ASTReader.cpp

@@ -2269,9 +2269,10 @@ ASTReader::ReadControlBlock(ModuleFile &F,
               (AllowConfigurationMismatch && Result == ConfigurationMismatch))
             Result = Success;
 
-          // If we've diagnosed a problem, we're done.
-          if (Result != Success &&
-              isDiagnosedResult(Result, ClientLoadCapabilities))
+          // If we can't load the module, exit early since we likely
+          // will rebuild the module anyway. The stream may be in the
+          // middle of a block.
+          if (Result != Success)
             return Result;
         } else if (Stream.SkipBlock()) {
           Error("malformed block record in AST file");

clang/test/Modules/implicit-build-config-out-of-date.m

@@ -0,0 +1,6 @@
+// RUN: rm -rf %t
+// Use -DA=0 so that there is at least one preprocessor option serialized after the diagnostic options.
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t -fimplicit-module-maps -I %S/Inputs %s -DA=0 -Rmodule-build -verify
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t -fimplicit-module-maps -I %S/Inputs %s -DA=0 -Werror -Rmodule-build -verify
+
+@import category_top; // expected-remark {{building module}} expected-remark {{finished building}}