Improve VFS compatibility on Windows

Keys in a virtual file system can be in Posix or Windows form or even
a combination of the two.  Many VFS tests (and a few Clang tests) were
XFAILed on Windows because of false negatives when comparing paths.

First, we default CaseSenstive to false on Windows.  This allows
drive letters like "D:" to match "d:".  Windows filesystems are, by
default, case insensitive, so this makes sense even beyond the drive
letter.

Second, we allow slashes to match backslashes when they're used as the
root component of a path.

Both of these changes are limited to RedirectingFileSystems, so there's
little chance of affecting other path handling.

These changes allow eleven of the VFS tests to pass on Windows as well
as three other Clang tests, so they have re-enabled.

This solves the majority of PR43272.  Additional VFS test failures will
be fixed in separate patches.

Differential Revision: https://reviews.llvm.org/D69958

include/llvm/Support/VirtualFileSystem.h

@@ -532,7 +532,7 @@ class RedirectingFileSystemParser;
 /// \endverbatim
 ///
 /// All configuration options are optional.
-///   'case-sensitive': <boolean, default=true>
+///   'case-sensitive': <boolean, default=(true for Posix, false for Windows)>
 ///   'use-external-names': <boolean, default=true>
 ///   'overlay-relative': <boolean, default=false>
 ///   'fallthrough': <boolean, default=true>
@@ -651,6 +651,17 @@ private:
     return ExternalFSValidWD && IsFallthrough;
   }
 
+  // In a RedirectingFileSystem, keys can be specified in Posix or Windows
+  // style (or even a mixture of both), so this comparison helper allows
+  // slashes (representing a root) to match backslashes (and vice versa).  Note
+  // that, other than the root, patch components should not contain slashes or
+  // backslashes.
+  bool pathComponentMatches(llvm::StringRef lhs, llvm::StringRef rhs) const {
+    if ((CaseSensitive ? lhs.equals(rhs) : lhs.equals_lower(rhs)))
+      return true;
+    return (lhs == "/" && rhs == "\\") || (lhs == "\\" && rhs == "/");
+  }
+
   /// The root(s) of the virtual file system.
   std::vector<std::unique_ptr<Entry>> Roots;
 
@@ -674,7 +685,12 @@ private:
   /// Whether to perform case-sensitive comparisons.
   ///
   /// Currently, case-insensitive matching only works correctly with ASCII.
-  bool CaseSensitive = true;
+  bool CaseSensitive =
+#ifdef _WIN32
+      false;
+#else
+      true;
+#endif
 
   /// IsRelativeOverlay marks whether a ExternalContentsPrefixDir path must
   /// be prefixed in every 'external-contents' when reading from YAML files.

lib/Support/VirtualFileSystem.cpp

@@ -1671,9 +1671,7 @@ RedirectingFileSystem::lookupPath(sys::path::const_iterator Start,
 
   // Forward the search to the next component in case this is an empty one.
   if (!FromName.empty()) {
-    if (CaseSensitive ? !Start->equals(FromName)
-                      : !Start->equals_lower(FromName))
-      // failure to match
+    if (!pathComponentMatches(*Start, FromName))
       return make_error_code(llvm::errc::no_such_file_or_directory);
 
     ++Start;
@@ -1695,6 +1693,7 @@ RedirectingFileSystem::lookupPath(sys::path::const_iterator Start,
     if (Result || Result.getError() != llvm::errc::no_such_file_or_directory)
       return Result;
   }
+
   return make_error_code(llvm::errc::no_such_file_or_directory);
 }