Clarify use of static inline functions in headers

In C++ it generally does not make sense to use static inline functions in headers.
If a function is outlined, 'inline' indicates that the function should be
deduplicated at link time (via COMDAT) and we do not end up with multiple
copies.

However, for the per-CPU "static inline" helpers we need "static" to avoid
ODR violations due to different labels emitted for the prologue for each
copy of the function.

PiperOrigin-RevId: 702759097
Change-Id: Icc82faadf30aa937e50a791c35a1de2d63dd77f7

tcmalloc/internal/optimization.h

@@ -56,7 +56,7 @@ namespace tcmalloc_internal {
 #define TCMALLOC_MUSTTAIL
 #endif
 
-static inline void* AssumeNotNull(void* p) {
+inline void* AssumeNotNull(void* p) {
   ASSUME(p != nullptr);
   return p;
 }

tcmalloc/internal/percpu_tcmalloc.h

@@ -599,6 +599,8 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void PrefetchSlabMemory(uintptr_t ptr) {
 }
 
 #if TCMALLOC_INTERNAL_PERCPU_USE_RSEQ && defined(__x86_64__)
+// Note: These helpers must be "static inline" to avoid ODR violations due to
+// different labels emitted in TCMALLOC_RSEQ_PROLOGUE.
 static inline ABSL_ATTRIBUTE_ALWAYS_INLINE bool TcmallocSlab_Internal_Push(
     size_t size_class, void* item) {
   uintptr_t scratch, current;