[clangd] Fix out-of-bounds read in packedLookup (#197021)

The `packedLookup` function doesn't work correctly with byte values over
0x7F (e.g. UTF-8 high bytes) on platforms where `char` is signed (like
x86-64). The character is treated as a negative `char`, which gets
converted to a negative `int`, which makes `I >> 2` negative, which
gives a negative index, and thus an out-of-bounds read.

The fix is to change the `int` parameter type to `unsigned char`, to
always get the value in the 0x00..0xFF range.

The issue has been discovered by the sanitizer buildbot after the
#187623 merge, which first introduced tests with non-ASCII source
content into the code-completion path.

clang-tools-extra/clangd/FuzzyMatch.cpp

@@ -145,7 +145,8 @@ constexpr static uint8_t CharRoles[] = {
     // clang-format on
 };
 
-template <typename T> static T packedLookup(const uint8_t *Data, int I) {
+template <typename T>
+static T packedLookup(const uint8_t *Data, unsigned char I) {
   return static_cast<T>((Data[I >> 2] >> ((I & 3) * 2)) & 3);
 }
 CharTypeSet calculateRoles(llvm::StringRef Text,

clang-tools-extra/clangd/unittests/FuzzyMatchTests.cpp

@@ -305,6 +305,8 @@ TEST(FuzzyMatch, Segmentation) {
               returns("+--+---+------"));
   EXPECT_THAT(segment("t3h PeNgU1N oF d00m!!!!!!!!"), //
               returns("+-- +-+-+-+ ++ +---        "));
+  EXPECT_THAT(segment("ab🙂cd"), //
+              returns("+-------"));
 }
 
 } // namespace