Cherry-picked from 40c7257 (without tests).

Rewrite -finitialize-locals to take an argument: no (default), c23,
c99, or c89 (case-insensitive).

- c23: uses = {} (C23 empty initializer)
- c99: uses designated initializers
- c89: uses positional initializers

Warning 30 (InitializerUnknownType) for unknown types, fatal by default.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Test suite in test/initialize-locals/ with standalone Makefile.
InitLocals.fst tests scalar, shadowed, branching, boolean, and struct
locals. Each test is compiled and executed with c23, c99, and c89 modes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

In the -fhoist-locals phase, comments in the declaration zone are now
hoisted alongside local variable declarations so that they maintain
their position relative to initialised declarations.  Previously, a
standalone comment could be swapped past a declaration with an
initializer because comments stayed in the residual body while
declarations were collected into the hoisted list.

Additionally, recurse into EIfThenElse, ESwitch, EWhile, EFor, and
EMatch bodies to hoist local variable declarations from inside those
blocks up to the function level, replacing each initialiser with an
assignment at the original location.

Buffer creations (EBufCreate) with constant size are split: the
declaration is hoisted with EAny as the initializer, and an EBufFill
is emitted at the original location (CStarToC11 renders it as memset
or a for-loop).  Likewise, EBufCreateL is split into a declaration
with all-EAny elements and individual EBufWrite statements at the
original location.  Buffer creations with non-constant size (VLA)
emit warning 29 and are left in place.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

When -fhoist-locals is enabled, local variable declarations are hoisted
to the function top. If a function contains #if/#else branches (from
IfDef-flagged externals), some hoisted variables may only be used in one
branch, triggering -Wunused-variable in the other.

Add a C11-level pass (MarkMaybeUnused) that detects variables not used
in all branches of an IfDef and inserts KRML_MAYBE_UNUSED_VAR() calls
after the declarations. This uses the existing KRML_MAYBE_UNUSED_VAR
macro which is available in both minimal and full krmllib headers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Instead of inserting KRML_MAYBE_UNUSED_VAR calls outside the IfDef
(between declarations and the #if), insert them at the top of each
branch for the variables that branch does not reference.

Before:
  uint32_t a; uint32_t b;
  KRML_MAYBE_UNUSED_VAR(a);
  KRML_MAYBE_UNUSED_VAR(b);
  #if USE_FEATURE
    a = ...; return a;
  #else
    b = ...; return b;
  #endif

After:
  uint32_t a; uint32_t b;
  #if USE_FEATURE
    KRML_MAYBE_UNUSED_VAR(b);
    a = ...; return a;
  #else
    KRML_MAYBE_UNUSED_VAR(a);
    b = ...; return b;
  #endif

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>