environment: update Debian development version in get_llvm_tool_names()

As far as we can ascertain, the idea here was supposed to be that this version
at the bottom gets updated as a last-resort for development LLVMs on Debian.

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>

mesonbuild/environment.py

@@ -209,7 +209,7 @@ def get_llvm_tool_names(tool: str) -> T.List[str]:
         '-3.7', '37',
         '-3.6', '36',
         '-3.5', '35',
-        '-15',    # Debian development snapshot
+        '-19',    # Debian development snapshot
         '-devel', # FreeBSD development snapshot
     ]
     names: T.List[str] = []