[LLVM] LTO usage errors incorrectly reported to the user as crashes/bugs

[bd1976bris]

Usage errors in LTOBackend.cpp are incorrectly reported to users as crashes/bugs.

For example, using the official LLVM 19 toolchain binaries, specifying an overly long cache directory results in the following output on my Windows 11 machine:

> "lld-link.exe" msvc.bc /lldltocache:<too long dir name> /entry:main /out:a.out
LLVM ERROR: can't create cache directory <too long dir name>: invalid argument

PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Exception Code: 0xC000001D
 #0 0x00007ff6b392cf36 xmlLinkGetData (c:\x\clang+llvm-19.1.0-x86_64-pc-windows-msvc\bin\lld-link.exe+0x180cf36)
 #1 0x00007ff6b38622d2 xmlLinkGetData (c:\x\clang+llvm-19.1.0-x86_64-pc-windows-msvc\bin\lld-link.exe+0x17422d2)
<...>

This kind of reporting is not appropriate for usage errors and recently confused one of Sony’s customers, who submitted a bug report (as instructed by the toolchain) after specifying an invalid LTO cache directory.

As another example, see the COFF LLD test lto-cache-errors.ll and note the use of not --crash in this test.

This issue is a bit more problematic on Windows than on Linux. On Windows, the use of reportFatalUsageError, which might be assumed to be a fix for this issue, produces a message asking users to report a bug.

Internal tracker: TOOLCHAIN-17744

[bd1976bris]

I have opened the following two PR's to address this issue:

1. Report usage errors in LTOBackend.cpp using reportFatalUsageError: [LLVM] Use reportFatalUsageError for LTO usage errors #140955
2. Don't emit crash-style error messages for usage errors on Windows: [LLVM][Windows] Elide PrettyStackTrace output for usage errors #140956

[bd1976bris]

Just noting that on Linux LLVM once called the signal handler for report_fatal_error but this changed in: 288999b.