[Parse] Disallow non-decimal line numbers to #sourceLocation directive

Non-decimal line numbers in a `#sourceLocation` directive are simply confusing in my opinion and should be banned.

Author: ahoppen

lib/Parse/ParseDecl.cpp

@@ -7186,6 +7186,11 @@ ParserStatus Parser::parseLineDirective(bool isLine) {
       }
       SmallString<16> buffer;
       auto text = stripUnderscoresIfNeeded(Tok.getText(), buffer);
+      if (text.find_first_not_of("0123456789") != StringRef::npos) {
+        // Disallow non-decimal line numbers in Swift 6.
+        diagnose(Tok, diag::expected_line_directive_number)
+            .warnUntilSwiftVersion(6);
+      }
       if (text.getAsInteger(0, StartLine)) {
         diagnose(Tok, diag::expected_line_directive_number);
         return makeParserError();

test/Parse/line-directive.swift

@@ -14,6 +14,9 @@ x // expected-error {{parameterless closing #sourceLocation() directive without
 
 #sourceLocation(file: x.swift, line: 1) // expected-error{{expected filename string literal}}
 
+// expected-warning@+1 {{expected starting line number}}
+#sourceLocation(file: "x.swift", line: 0xff)
+
 #sourceLocation(file: "x.swift", line: 42)
 x x ; // should be ignored by expected_error because it is in a different file
 x