[CodeGen][i386] Move -mregparm storage earlier and fix Runtime calls (#89707)

When building the Linux kernel for i386, the -mregparm=3 option is
enabled. Crashes were observed in the sanitizer handler functions, and
the problem was found to be mismatched calling convention.

As was fixed in commit c167c0a4 ("[BuildLibCalls] infer inreg param
attrs from NumRegisterParameters"), call arguments need to be marked as
"in register" when -mregparm is set. Use the same helper developed there
to update the function arguments.

Since CreateRuntimeFunction() is actually part of CodeGenModule, storage
of the -mregparm value is also moved to the constructor, as doing this
in Release() is too late.

Fixes: https://github.com/llvm/llvm-project/issues/89670