[clang][lex] Fix failures with Microsoft header search rules

[jansvoboda11]

Summary

HeaderSearch currently assumes LookupFileCache is eventually populated in LookupFile. However, that's not always the case with -fms-compatibility and its early returns.

This patch adds a defensive check that the iterator pulled out of the cache is actually valid before using it.

(This bug was introduced in D119721. Before that, the cache was initialized to 0 - essentially the search_dir_begin() iterator.)

[erichkeane]

I think this ends up being the same as what we came up with internally, so I think this LGTM!

[dexonsmith]

LGTM.

[jansvoboda11]

Remove slash from test to make it pass on Windows

clang/lib/Lex/HeaderSearch.cpp

Optional<FileEntryRef> HeaderSearch::LookupFile(
 
   ConstSearchDirIterator NextIt = std::next(It);
 
   // If the entry has been previously looked up, the first value will be
   // non-zero.  If the value is equal to i (the start point of our search), then
   // this is a matching hit.
   if (!SkipCache && CacheLookup.StartIt == NextIt) {
     // Skip querying potentially lots of directories for this lookup.
+    if (CacheLookup.HitIt)
       It = CacheLookup.HitIt;
     if (CacheLookup.MappedName) {
       Filename = CacheLookup.MappedName;
       if (IsMapped)
         *IsMapped = true;
     }
   } else {
     // Otherwise, this is the first query, or the previous query didn't match
     // our search start.  We will fill in our found location below, so prime the

clang/test/Preprocessor/microsoft-header-search-fail.c

+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -Eonly -fms-compatibility %t/test.c -I %t/include -verify
+
+//--- test.c
+#include "x/header.h"
+#include "z/header.h"
+
+// expected-warning-re@include/y/header.h:1 {{#include resolved using non-portable Microsoft search rules as: {{.*}}x/culprit.h}}
+// expected-error@include/z/header.h:1 {{'culprit.h' file not found}}
+
+//--- include/x/header.h
+#include "y/header.h"
+
+//--- include/y/header.h
+#include "culprit.h"
+
+//--- include/x/culprit.h
+
+//--- include/z/header.h
+#include "culprit.h"