Revert "[Modules] Prevent #import to reenter header if not building a module."

This reverts commit r310605. Richard pointed out a better way to achieve
this, which I'll post a patch for soon.

llvm-svn: 310775
diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index 4b5df8c..1ebcc0a 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1143,7 +1143,7 @@
     // headers find in the wild might rely only on #import and do not contain
     // controlling macros, be conservative and only try to enter textual headers
     // if such macro is present.
-    if (FileInfo.isCompilingModuleHeader && !FileInfo.isModuleHeader &&
+    if (!FileInfo.isModuleHeader &&
         FileInfo.getControllingMacro(ExternalLookup))
       TryEnterHdr = true;
     return TryEnterHdr;