[CIR] Implement Namespace/global TLS CIR CodeGen (#196332)
Unlike local TLS, global TLS functions need to be initialized upon their
first use in a thread.
First, all attempts to 'get' said TLS global are replaced with calls to
a 'wrapper' function, which calls an 'init' alias function, then returns
the global. While classic codegen manages to omit this in simple cases
sometimes, this CIR implementation doesn't attempt to do such constant
folding/inlining. The call to the 'init' is omitted if there is no
ctor/dtor setup required, so sometimes the wrapper is just a 'no-op'
(intentionally!).
There are also two types of 'global' TLS functions: unordered, and
ordered. Unordered are typically variable templates, and their 'init'
function initializes JUST them. The rest are ordered, which requires all
ordered initializations to happen as soon as any happen.
The Wrapper:
If necessary (omitted in a few places), calls the 'init' alias, then
returns the global.
Global Init Function (__tls_init):
For ordered global TLS, a use of 1 TLS is required to initialize ALL of
them. This function checks the global guard (__tls_guard), and calls the
individual 'init' functions for each global(__cxx_global_var_init).
Unordered ones do not get entered here.
Individual init Functions (__cxx_global_var_init): Like the rest of our
globals, these emit a __cxx_global_var_init function. However, they are
not added to the global constructors list. These are identically emitted
with one exception: Unordered Global TLS variables wrap said init in a
guard check (ordered have their's guarded by __tls_guard in __tls_init).
Init Alias:
This alias is called by the wrapper function to make sure said variable
is initialized before use. IN the case of ordered globals, this is an
alias to __tls_init. IN the case of unordered globals, this goes
directly to the __cxx_global_var_init for that variable.
This patch implements the codegen part of the above by introducing a set
of 3 strings on a GlobalOp in an attribute. if present, this will cause
the above global tls behavior (with the next patch). At the moment, this
only generates the attribute, and the lowering-prepare patches will come
in future patches.
