[ClangImporter] Allow importing modules even if there's a fatal Clang error.

This was supposed to prevent crashes, but it ended up /causing/ crashes
because we may hit a fatal Clang error /while/ importing Clang modules
and bridging headers.

I left the test on the logic that imports bridging headers because that
happens under more controlled circumstances. (Clang modules that have
Swift counterparts should never have bridging headers.)

rdar://problem/21689014

Swift SVN r30512

Author: jrose-apple

lib/ClangImporter/ClangImporter.cpp

@@ -827,13 +827,7 @@ std::string ClangImporter::getBridgingHeaderContents(StringRef headerPath,
 Module *ClangImporter::loadModule(
     SourceLoc importLoc,
     ArrayRef<std::pair<Identifier, SourceLoc>> path) {
-  // Don't even try to load the module if the Clang context is in a bad way.
-  // It could cause a crash.
   auto &clangContext = Impl.getClangASTContext();
-  auto &clangDiags = clangContext.getDiagnostics();
-  if (clangDiags.hasFatalErrorOccurred())
-    return nullptr;
-
   auto &clangHeaderSearch = Impl.getClangPreprocessor().getHeaderSearchInfo();
 
   // Look up the top-level module first, to see if it exists at all.

test/ClangModules/sdk-bridging-header.swift

@@ -1,4 +1,7 @@
 // RUN: %target-swift-frontend -parse -verify %s -import-objc-header %S/Inputs/sdk-bridging-header.h
+// RUN: not %target-swift-frontend -parse %s -import-objc-header %S/Inputs/bad-bridging-header.h 2>&1 | FileCheck -check-prefix=CHECK-FATAL %s
+
+// CHECK-FATAL: failed to import bridging header
 
 // REQUIRES: objc_interop