Collect built-in LWLock tranche names statically, not dynamically.

There is little point in using the LWLockRegisterTranche mechanism for
built-in tranche names.  It wastes cycles, it creates opportunities for
bugs (since failing to register a tranche name is a very hard-to-detect
problem), and the lack of any centralized list of names encourages
sloppy nonconformity in name choices.  Moreover, since we have a
centralized list of the tranches anyway in enum BuiltinTrancheIds, we're
certainly not buying any flexibility in return for these disadvantages.

Hence, nuke all the backend-internal LWLockRegisterTranche calls,
and instead provide a const array of the builtin tranche names.

(I have in mind to change a bunch of these names shortly, but this
patch is just about getting them into one place.)

Discussion: https://postgr.es/m/9056.1589419765@sss.pgh.pa.us

src/backend/storage/buffer/buf_init.c

@@ -87,9 +87,6 @@ InitBufferPool(void)
 						NBuffers * (Size) sizeof(LWLockMinimallyPadded),
 						&foundIOLocks);
 
-	LWLockRegisterTranche(LWTRANCHE_BUFFER_IO_IN_PROGRESS, "buffer_io");
-	LWLockRegisterTranche(LWTRANCHE_BUFFER_CONTENT, "buffer_content");
-
 	/*
 	 * The array used to sort to-be-checkpointed buffer ids is located in
 	 * shared memory, to avoid having to allocate significant amounts of