[libclc] [libclc] Restore always_inline for SPIR-V targets (PR #202891)

[Wenju He via cfe-commits]

wenju-he wrote:

alwaysinline on every function can bloat code size in some cases. Does NIR have set up inline threshold for proper inlining?
This PR probably is against the motivation in #158791
SPIRV is a generic target, even though it is currently only used in Mesa. If its behavior of _CLC_DEF and _CLC_INLINE change, then all targets should change as well.

https://github.com/llvm/llvm-project/pull/202891