meson: Add some missing LLVM function checks

The checks for

HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER and
HAVE_DECL_LLVMCREATEPERFJITEVENTLISTENER

are in configure but are missing on the meson side.  This adds those.

Reported-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/5539b16c-cff7-46d5-9621-c3fb6b549e9e@iki.fi

meson.build

@@ -2301,6 +2301,14 @@ decl_checks += [
   ['pwritev', 'sys/uio.h'],
 ]
 
+# Check presence of some optional LLVM functions.
+if llvm.found()
+  decl_checks += [
+    ['LLVMCreateGDBRegistrationListener', 'llvm-c/ExecutionEngine.h'],
+    ['LLVMCreatePerfJITEventListener', 'llvm-c/ExecutionEngine.h'],
+  ]
+endif
+
 foreach c : decl_checks
   func = c.get(0)
   header = c.get(1)