Disable Clang optimizations for Session::get*LibraryCode() functions

[ncelikNV]

Clang was taking around 10 minutes to compile
slang-embedded-core-module-source.cpp with optimizations. Disabling
optimizations for these functions reduced the compilation time to about
12 seconds on my machine (Intel Core Ultra 7 165H), and had no
noticeable impact on run-time performance.

Run-time performance with optimizations:

$ hyperfine --shell=none './build/generators/Release/bin/slang-bootstrap -archive-type riff-lz4 -save-core-module-bin-source slang-core-module-generated.h -save-glsl-module-bin-source slang-glsl-module-generated.h'
Benchmark 1: ./build/generators/Release/bin/slang-bootstrap -archive-type riff-lz4 -save-core-module-bin-source slang-core-module-generated.h -save-glsl-module-bin-source slang-glsl-module-generated.h
  Time (mean ± σ):      2.545 s ±  0.035 s    [User: 2.333 s, System: 0.210 s]
  Range (min … max):    2.496 s …  2.620 s    10 runs

Run-time performance without optimizations:

$ hyperfine --shell=none './build/generators/Release/bin/slang-bootstrap -archive-type riff-lz4 -save-core-module-bin-source slang-core-module-generated.h -save-glsl-module-bin-source slang-glsl-module-generated.h'
Benchmark 1: ./build/generators/Release/bin/slang-bootstrap -archive-type riff-lz4 -save-core-module-bin-source slang-core-module-generated.h -save-glsl-module-bin-source slang-glsl-module-generated.h
  Time (mean ± σ):      2.564 s ±  0.039 s    [User: 2.350 s, System: 0.213 s]
  Range (min … max):    2.512 s …  2.614 s    10 runs

Disabling optimizations also makes
slang-embedded-core-module-source.cpp.o slightly smaller:

• 7.84 MiB with optimizations,
• 7.37 MiB without optimizations.

Fixes #9054.

[jkwak-work]

I think we still want to enable the optimization for the release package and anybody building Slang from source code.
This optimization should be applied to Slang dev members who compiles frequently.
And it should be gated by CMake build flag as described in the issue:

• slang-embedded-core-module-source.cpp takes long to compile with Clang 18 & 20 #9054

[ncelikNV]

These functions are only called in the following cases:

• By slang-bootstrap when building Slang.
• By slangd at start-up if --print-builtin-module was passed as command-line argument.
• At run time if -DSLANG_EMBED_CORE_MODULE=OFF was passed to CMake when building Slang. Do we know if any users disable this CMake option?

As I've shown, disabling optimizations for these functions has no noticeable impact on slang-bootstrap's execution time, and code size isn't lower with optimizations either.

Clang takes 11-15x as long as GCC to compile this file, and >90% of the compilation time is spent on one single LLVM optimization pass. There is something wrong with LLVM here. See llvm/llvm-project#132554.

Why make this more complex just so we can only optionally work around a bug in LLVM, when the workaround shows no downsides?

[jkwak-work]

I see the point.
This part of code doesn't really need to be optimized because it will run to generate the slang-XXX-module.bin only once.
Let's go with what we have here.

[jkwak-work]

Looks good to me.

Let's go with this simpler approach rather than adding a new CMake option.

[jkiviluoto-nv]

LGTM but stating the runtime impact in comment makes sense.

[jkiviluoto-nv]

LGTM