Bug 1573501 - Disable -Wtautological-type-limit-compare r=froydnj

This warning, new in clang 9, is noisy about patterns like:

```
unsigned long x = ...
if (x > 4294967295)
```

The condition is always false on 32-bit and LLP64 systems, but on LP64 it's a valid test.

Differential Revision: https://phabricator.services.mozilla.com/D42346

--HG--
extra : moz-landing-system : lando

build/moz.configure/warnings.configure

@@ -109,6 +109,8 @@ check_and_add_gcc_warning('-Wstring-conversion')
 check_and_add_gcc_warning('-Wtautological-overlap-compare')
 check_and_add_gcc_warning('-Wtautological-unsigned-enum-zero-compare')
 check_and_add_gcc_warning('-Wtautological-unsigned-zero-compare')
+# This can be triggered by certain patterns used deliberately in portable code
+check_and_add_gcc_warning('-Wno-error=tautological-type-limit-compare')
 
 # we inline 'new' and 'delete' in mozalloc
 check_and_add_gcc_warning('-Wno-inline-new-delete', cxx_compiler)