diff --git a/clang/include/clang/Basic/Sanitizers.h b/clang/include/clang/Basic/Sanitizers.h
--- a/clang/include/clang/Basic/Sanitizers.h
+++ b/clang/include/clang/Basic/Sanitizers.h
@@ -16,7 +16,6 @@
 
 #include "clang/Basic/LLVM.h"
 #include "llvm/ADT/StringRef.h"
-#include "llvm/Support/MathExtras.h"
 #include "llvm/Transforms/Instrumentation/AddressSanitizerOptions.h"
 #include <cassert>
 #include <cstdint>
@@ -60,12 +59,7 @@
     return SanitizerMask(mask1, mask2);
   }
 
-  unsigned countPopulation() const {
-    unsigned total = 0;
-    for (const auto &Val : maskLoToHigh)
-      total += llvm::countPopulation(Val);
-    return total;
-  }
+  unsigned countPopulation() const;
 
   void flipAllBits() {
     for (auto &Val : maskLoToHigh)
diff --git a/clang/lib/Basic/Sanitizers.cpp b/clang/lib/Basic/Sanitizers.cpp
--- a/clang/lib/Basic/Sanitizers.cpp
+++ b/clang/lib/Basic/Sanitizers.cpp
@@ -14,6 +14,7 @@
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/Support/MathExtras.h"
 
 using namespace clang;
 
@@ -57,6 +58,13 @@
 }
 
 namespace clang {
+unsigned SanitizerMask::countPopulation() const {
+  unsigned total = 0;
+  for (const auto &Val : maskLoToHigh)
+    total += llvm::countPopulation(Val);
+  return total;
+}
+
 llvm::hash_code hash_value(const clang::SanitizerMask &Arg) {
   return Arg.hash_value();
 }