[turbofan] Remove defn of constexpr vars for C++17

It's deprecated in C++17 and not needed (it was needed for static const,
but not for constexpr). It's needed in C++14, but can trigger
deprecation errors in C++17, so include it conditionally on __cplusplus.

Bug: chromium:1274247
Change-Id: I92524f377bb92ac2aa008f15830920786a1c1432
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3308795
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78157}

include/v8config.h

@@ -591,6 +591,13 @@ V8 shared library set USING_V8_SHARED.
 #define V8_CAGED_POINTERS
 #endif
 
+// From C++17 onwards, static constexpr member variables are defined to be
+// "inline", and adding a separate definition for them can trigger deprecation
+// warnings. For C++14 and below, however, these definitions are required.
+#if __cplusplus < 201703L
+#define V8_STATIC_CONSTEXPR_VARIABLES_NEED_DEFINITIONS
+#endif
+
 // clang-format on
 
 #undef V8_HAS_CPP_ATTRIBUTE

src/compiler/js-heap-broker.cc

@@ -30,10 +30,13 @@ namespace compiler {
 
 #define TRACE(broker, x) TRACE_BROKER(broker, x)
 
+#ifdef V8_STATIC_CONSTEXPR_VARIABLES_NEED_DEFINITIONS
 // These definitions are here in order to please the linker, which in debug mode
 // sometimes requires static constants to be defined in .cc files.
+// This is, however, deprecated (and unnecessary) in C++17.
 const uint32_t JSHeapBroker::kMinimalRefsBucketCount;
 const uint32_t JSHeapBroker::kInitialRefsBucketCount;
+#endif
 
 void JSHeapBroker::IncrementTracingIndentation() { ++trace_indentation_; }