Add a function to MD5 a file's contents.

In doing so, clean up the MD5 interface a little.  Most
existing users only care about the lower 8 bytes of an MD5,
but for some users that care about the upper and lower,
there wasn't a good interface.  Furthermore, consumers
of the MD5 checksum were required to handle endianness
details on their own, so it seems reasonable to abstract
this into a nicer interface that just gives you the right
value.

Differential Revision: https://reviews.llvm.org/D31105

llvm-svn: 298322
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 952992a..2acdc64 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -1252,7 +1252,7 @@
   PreambleFileHash Result;
   Result.Size = Size;
   Result.ModTime = ModTime;
-  memset(Result.MD5, 0, sizeof(Result.MD5));
+  Result.MD5 = {};
   return Result;
 }
 
@@ -1273,7 +1273,7 @@
 bool operator==(const ASTUnit::PreambleFileHash &LHS,
                 const ASTUnit::PreambleFileHash &RHS) {
   return LHS.Size == RHS.Size && LHS.ModTime == RHS.ModTime &&
-         memcmp(LHS.MD5, RHS.MD5, sizeof(LHS.MD5)) == 0;
+         LHS.MD5 == RHS.MD5;
 }
 } // namespace clang