Port TS PR 59803 (Remove redundant getStringLiteralType call) (#1254)

Author: jakebailey

internal/checker/relater.go

@@ -2442,7 +2442,7 @@ func (c *Checker) isValidTypeForTemplateLiteralPlaceholder(source *Type, target
 		return target.flags&TypeFlagsNumber != 0 && isValidNumberString(value, false /*roundTripOnly*/) ||
 			target.flags&TypeFlagsBigInt != 0 && isValidBigIntString(value, false /*roundTripOnly*/) ||
 			target.flags&(TypeFlagsBooleanLiteral|TypeFlagsNullable) != 0 && value == target.AsIntrinsicType().intrinsicName ||
-			target.flags&TypeFlagsStringMapping != 0 && c.isMemberOfStringMapping(c.getStringLiteralType(value), target) ||
+			target.flags&TypeFlagsStringMapping != 0 && c.isMemberOfStringMapping(source, target) ||
 			target.flags&TypeFlagsTemplateLiteral != 0 && c.isTypeMatchedByTemplateLiteralType(source, target.AsTemplateLiteralType())
 	case source.flags&TypeFlagsTemplateLiteral != 0:
 		texts := source.AsTemplateLiteralType().texts