SourceManager::GetDefaultFileAndLine locates the executable's main to
pick a default source file, used for input like "break set -l N" with no
file specified. It searched for main by full name, but a function's
linkage name can differ from its source name, in which case a full-name
search for "main" does not find it and no default file is chosen.
For example, wasi-libc renames main to __main_argc_argv, so on
WebAssembly the debug function for main was never matched and "break set
-l N" failed with "no default file available".
When we can't find "main", fall back to searching by base name.
Jonas Devlieghere
0a2fb2a226