[DebugNames] Compare TableEntry names more efficiently (#79759)

TableEntry names are pointers into the string table section, and
accessing their
length requires a search for `\0`. However, 99% of the time we only need
to
compare the name against some other other, and such a comparison will
fail as
early as the first character.

This commit adds a method to the interface of TableEntry so that such a
comparison can be done without extracting the full name. It saves 10% in
the
time (1250ms -> 1100 ms) to evaluate the following expression.

```
lldb \
  --batch \
  -o "b CodeGenFunction::GenerateCode" \
  -o run \
  -o "expr Fn" \
  -- \
  clang++ -c -g test.cpp -o /dev/null &> output
```