[rust] Remove constexpr from functions that call non-constexpr functions

The CHECK macros will call non-constexpr functions on failure. While
this is legal C++14 as long as the function doesn't actually get called,
GCC 5.4 will choke on it.

Author: nikic

compiler-rt/lib/sanitizer_common/sanitizer_common.h

@@ -443,7 +443,7 @@ inline uptr RoundUpToPowerOfTwo(uptr size) {
   return 1ULL << (up + 1);
 }
 
-inline constexpr uptr RoundUpTo(uptr size, uptr boundary) {
+inline uptr RoundUpTo(uptr size, uptr boundary) {
   RAW_CHECK(IsPowerOfTwo(boundary));
   return (size + boundary - 1) & ~(boundary - 1);
 }

compiler-rt/lib/sanitizer_common/sanitizer_stack_store.h

@@ -61,7 +61,7 @@ class StackStore {
     return frame_idx % kBlockSizeFrames;
   }
 
-  static constexpr uptr IdToOffset(Id id) {
+  static uptr IdToOffset(Id id) {
     CHECK_NE(id, 0);
     return id - 1;  // Avoid zero as id.
   }