[clang-query] Continue if compilation command not found for some files

When searching for a code pattern in an entire project with a
compilation database it's tempting to run

```
clang-query **.cpp
```

And yet, that often breaks because some files are just not in the
compilation database: tests, sample code, etc..
clang-query should not stop when encountering such cases.

Differential Revision: https://reviews.llvm.org/D51183

llvm-svn: 348328

clang-tools-extra/clang-query/tool/ClangQuery.cpp

@@ -100,8 +100,19 @@ int main(int argc, const char **argv) {
   ClangTool Tool(OptionsParser.getCompilations(),
                  OptionsParser.getSourcePathList());
   std::vector<std::unique_ptr<ASTUnit>> ASTs;
-  if (Tool.buildASTs(ASTs) != 0)
+  int Status = Tool.buildASTs(ASTs);
+  int ASTStatus = 0;
+  if (Status == 1) {
+    // Building ASTs failed.
     return 1;
+  } else if (Status == 2) {
+    ASTStatus |= 1;
+    llvm::errs() << "Failed to build AST for some of the files, "
+                 << "results may be incomplete."
+                 << "\n";
+  } else {
+    assert(Status == 0 && "Unexpected status returned");
+  }
 
   QuerySession QS(ASTs);
 
@@ -134,5 +145,5 @@ int main(int argc, const char **argv) {
     }
   }
 
-  return 0;
+  return ASTStatus;
 }