[clang] [outliner] Make `nooutline` accessible from `clang`

[thughes]

When LTO is enabled, outlining is performed. Right now the outliner is all-or-nothing, which is not always desirable. For example, consider an ASSERT macro that has instructions to generate a processor exception. We may not want this code to be outlined, since that would result in the PC reported by the exception handler to be inside the outlined function, rather than the function calling the ASSERT.

Can we make nooutline accessible from clang to disable specific statements/functions from being outlined?

[gburgessiv]

Thanks for filing!

It looks like we also have -mno-outline, which doesn't turn into fn metadata? Seems like that would lead to:

$ clang -O3 -flto=thin -mno-outline -c foo.c -o foo.o
$ clang -O3 -flto=thin -c bar.c -o bar.o
$ clang -O3 -flto=thin foo.o bar.o -o exe
# oops, outlining was done on both `bar.c` and `foo.c`'s functions 

...which is probably a bug.

[gburgessiv]

/cc @kyulee-com & @xuanzhang816, as git says they've done significant outliner work