clang-cl: Flush stdout after writing the /showIncludes output

[• ehsan]

Summary

Before this patch, you could get lines in the output such as:
Note: including file: ../../dist/include/js/Tc:/path/to/foo.cpp(1,1) : error(clang): static_assert failed...

This patch ensures that the stdout output from showIncludes won't be garbled
in the terminal like this, and it also helps applications that use the output
to generate dependency information if they happen to capture both stdout and
stderr.

[• ehsan]

Note that I didn't make this flush conditional on MSStyle since I think it's good practice to flush here even for non-clang-cl mode where we output to stderr, but I'd be happy to change that if you think that is not appropriate.

[thakis]

lgtm

(I can't think of a great way to test this.)

[• ehsan]

Closed by commit rL213297 (authored by @ehsan).

lib/Frontend/HeaderIncludeGen.cpp

if (ShowDepth) {
 
       if (!MSStyle)
         Msg += ' ';
     }
     Msg += Filename;
     Msg += '\n';
 
     OutputFile->write(Msg.data(), Msg.size());
+    OutputFile->flush();
   }
 }