[clang][docs] Clarify the semantics of -fexceptions

[asb]

Summary

As noted in #6126 the documentation for -fexceptions appears to imply that unwind information is always generated, which isn't the case.

[efriedma]

I'd prefer to focus the documentation more generically on exceptions, not unwind information specifically. -fexceptions makes code generation change in other ways, and "unwind information" doesn't exist on all targets where exception handling is relevant.

[asb]

What do you think about e.g. "Allow exceptions to be thrown through Clang compiled stack frames (on many targets, this will enable unwind information for functions that might have an exception thrown through them). This is on by default in x86-64"? Or were you thinking of something else?

[efriedma]

Something like that, sure.

Is it actually on by default on x86-64? I don't think the default is target-specific, I think it's just on for default for C++ dialects, and off by default for non-C++ dialects.

[asb]

I don't see anything x86_64 specific for OPT_fexceptions handling in clang/lib/Driver/ToolChains/Clang.cpp. So perhaps "For most targets, this is enabled by default for C++." It's disabled for the ps4/ps5 toolchains and xcore, hence the "most targets" qualifier.

[asb]

Adjust wording based on discussion.

[efriedma]

LGTM

clang/docs/CommandGuide/clang.rst

 .. option:: -feliminate-unused-debug-types
 
   By default, Clang does not emit type information for types that are defined
   but not used in a program. To retain the debug info for these unused types,
   the negation **-fno-eliminate-unused-debug-types** can be used.
 
 .. option:: -fexceptions
 
-  Enable generation of unwind information. This allows exceptions to be thrown
-  through Clang compiled stack frames.  This is on by default in x86-64.
+  Allow exceptions to be thrown through Clang compiled stack frames (on many
+  targets, this will enable unwind information for functions that might have
+  an exception thrown through them). For most targets, this is enabled by
+  default for C++.
 
 .. option:: -ftrapv
 
   Generate code to catch integer overflow errors.  Signed integer overflow is
   undefined in C. With this flag, extra code is generated to detect this and
   abort when it happens.
 
 .. option:: -fvisibility