Turn special page pointer validation to static inline function

Inclusion of multiple macros inside another macro was pushing MSVC
past its size liimit.  Reported by buildfarm.

src/include/storage/bufpage.h

@@ -297,15 +297,32 @@ typedef PageHeaderData *PageHeader;
 #define PageGetSpecialSize(page) \
 	((uint16) (PageGetPageSize(page) - ((PageHeader)(page))->pd_special))
 
+/*
+ * Using assertions, validate that the page special pointer is OK.
+ *
+ * This is intended to catch use of the pointer before page initialization.
+ * It is implemented as a function do to the limitations of the MSVC compiler,
+ * which choked on doing all these tests within another macro.  We return true
+ * so that MacroAssert() can be used while still getting the specifics from
+ * the macro failure within this function.
+ */
+static inline bool
+PageValidateSpecialPointer(Page page)
+{
+	Assert(PageIsValid(page));
+	Assert(((PageHeader) (page))->pd_special <= BLCKSZ);
+	Assert(((PageHeader) (page))->pd_special >= SizeOfPageHeaderData);
+
+	return true;
+}
+
 /*
  * PageGetSpecialPointer
  *		Returns pointer to special space on a page.
  */
 #define PageGetSpecialPointer(page) \
 ( \
-	AssertMacro(PageIsValid(page)), \
+	AssertMacro(PageValidateSpecialPointer(page)), \
-	AssertMacro(((PageHeader) (page))->pd_special <= BLCKSZ), \
-	AssertMacro(((PageHeader) (page))->pd_special >= SizeOfPageHeaderData), \
 	(char *) ((char *) (page) + ((PageHeader) (page))->pd_special) \
 )