Convert *GetDatum() and DatumGet*() macros to inline functions

The previous macro implementations just cast the argument to a target
type but did not check whether the input type was appropriate.  The
function implementation can do better type checking of the input type.

For the *GetDatumFast() macros, converting to an inline function
doesn't work in the !USE_FLOAT8_BYVAL case, but we can use
AssertVariableIsOfTypeMacro() to get a similar level of type checking.

Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/8528fb7e-0aa2-6b54-85fb-0c0886dbd6ed%40enterprisedb.com

src/backend/access/gist/gistutil.c

@@ -280,7 +280,7 @@ gistMakeUnionKey(GISTSTATE *giststate, int attno,
 bool
 gistKeyIsEQ(GISTSTATE *giststate, int attno, Datum a, Datum b)
 {
-	bool		result;
+	bool		result = false; /* silence compiler warning */
 
 	FunctionCall3Coll(&giststate->equalFn[attno],
 					  giststate->supportCollation[attno],

src/backend/tsearch/ts_parse.c

@@ -354,7 +354,7 @@ void
 parsetext(Oid cfgId, ParsedText *prs, char *buf, int buflen)
 {
 	int			type,
-				lenlemm;
+				lenlemm = 0;	/* silence compiler warning */
 	char	   *lemm = NULL;
 	LexizeData	ldata;
 	TSLexeme   *norms;
@@ -529,7 +529,7 @@ void
 hlparsetext(Oid cfgId, HeadlineParsedText *prs, TSQuery query, char *buf, int buflen)
 {
 	int			type,
-				lenlemm;
+				lenlemm = 0;	/* silence compiler warning */
 	char	   *lemm = NULL;
 	LexizeData	ldata;
 	TSLexeme   *norms;

src/backend/utils/mb/mbutils.c

@@ -409,8 +409,8 @@ pg_do_encoding_conversion(unsigned char *src, int len,
 	(void) OidFunctionCall6(proc,
 							Int32GetDatum(src_encoding),
 							Int32GetDatum(dest_encoding),
-							CStringGetDatum(src),
-							CStringGetDatum(result),
+							CStringGetDatum((char *) src),
+							CStringGetDatum((char *) result),
 							Int32GetDatum(len),
 							BoolGetDatum(false));
 
@@ -485,8 +485,8 @@ pg_do_encoding_conversion_buf(Oid proc,
 	result = OidFunctionCall6(proc,
 							  Int32GetDatum(src_encoding),
 							  Int32GetDatum(dest_encoding),
-							  CStringGetDatum(src),
-							  CStringGetDatum(dest),
+							  CStringGetDatum((char *) src),
+							  CStringGetDatum((char *) dest),
 							  Int32GetDatum(srclen),
 							  BoolGetDatum(noError));
 	return DatumGetInt32(result);
@@ -910,8 +910,8 @@ pg_unicode_to_server(pg_wchar c, unsigned char *s)
 	FunctionCall6(Utf8ToServerConvProc,
 				  Int32GetDatum(PG_UTF8),
 				  Int32GetDatum(server_encoding),
-				  CStringGetDatum(c_as_utf8),
-				  CStringGetDatum(s),
+				  CStringGetDatum((char *) c_as_utf8),
+				  CStringGetDatum((char *) s),
 				  Int32GetDatum(c_as_utf8_len),
 				  BoolGetDatum(false));
 }