[Clang] Fix invalid utf-8 detection

[cor3ntin]

Summary

The length of valid codepoints was incorrectly
calculated which was not caught before because the
absence of tests for the valid codepoints scenario.

[thakis]

This breaks check-clang: http://45.33.8.238/linux/80462/step_7.txt

Please take a look and revert for now if it takes a while to fix.

[thakis]

Reverted in e9fe20dab39edb911afacdb6ed9ec2c7499a59cf for now.

[thakis]

In D129223#3633754, @thakis wrote:

This breaks check-clang: http://45.33.8.238/linux/80462/step_7.txt

(Also on all other bots, e.g. https://lab.llvm.org/buildbot/#/builders/171/builds/17060 – did you run tests locally before committing?)

[cor3ntin]

In D129223#3633849, @thakis wrote:

Reverted in e9fe20dab39edb911afacdb6ed9ec2c7499a59cf for now.

Thanks, I was going to do that when I noticed you already did, very much appreciated.
The patch (well, the one before) appeared to have multiple off-by one errors that cancelled each other and were not caught due to insufficient test cases
Once I find a fix, I'll get it reviewed again. Sorry for the inconvenience

clang/test/Lexer/comment-invalid-utf8.c

 /*
 €
 */
 // expected-warning@-2 {{invalid UTF-8 in comment}}
 
 // abcd
 // €abcd
 // expected-warning@-1 {{invalid UTF-8 in comment}}
+
+
+//§ § § 😀 ä½!好 ©
+
+/*§ § § 😀 ä½!好 ©*/
+
+/*
+§ § § 😀 ä½!好 ©
+*/
+
+/* § § § 😀 ä½!好 © */

llvm/lib/Support/ConvertUTF.cpp

 }
 
 /*
  * Exported function to return the size of the first utf-8 code unit sequence,
  * Or 0 if the sequence is not valid;
  */
 unsigned getUTF8SequenceSize(const UTF8 *source, const UTF8 *sourceEnd) {
   int length = trailingBytesForUTF8[*source] + 1;
-  return (length > sourceEnd - source && isLegalUTF8(source, length)) ? length
+  return (length < sourceEnd - source && isLegalUTF8(source, length)) ? length
                                                                       : 0;
 }
 
 /* --------------------------------------------------------------------- */
 
 static unsigned
 findMaximalSubpartOfIllFormedUTF8Sequence(const UTF8 *source,
                                           const UTF8 *sourceEnd) {