[Metal] Carry export name separately from MSL entry point for name lookup (#24642)

spirv-cross rewrites SPIR-V entry point names to be legal MSL/C
identifiers when serializing Metal executables: '.' characters (codegen
dispatch names like _dot_general_matmul_i32.i32.i32_...) become '_', and
reserved '__' prefixes (stream builtins like __builtin_splat_i64) are
dropped. The MetalSPIRV target stored only the rewritten MSL function
name in PipelineDef.entry_point, and the runtime loaded pipeline->name
from that same field -- so hal.executable.lookup.function, which
references the original un-rewritten hal.executable.export symbol name,
could never match for any export whose name spirv-cross rewrites. This
broke every metal-spirv e2e test using such exports
(dot/dot_general/convolution matmul kernels and
compare/dynamic_slice/gather stream builtins).

Add a dedicated always-present name field to PipelineDef carrying the
canonical hal.executable.export symbol name (populated from
exportOp.getName() in the compiler), appended at the end of the table
for backward-compatible vtable layout. The runtime now sources
pipeline->name (used by lookup_function_by_name) from this field,
keeping entry_point as the MTLFunction name for -[MTLLibrary
newFunctionWithName:]. The loader falls back to entry_point when name is
absent for compatibility.

Verified: dot_general (8/8), compare (15/15), and the full Metal CTS
suite
(command_buffer_tests/queue_tests/dispatch_tests/queue_dispatch_tests,
4/4) pass. Signed-off-by: Alex Vasile
<48962821+Alex-Vasile@users.noreply.github.com>

Signed-off-by: Alex Vasile <48962821+Alex-Vasile@users.noreply.github.com>