[X86][regcall] Rework struct classification for non-Windows x86-64 targets (#187134)

Currently, when `X86_64ABIInfo::classifyRegCallStructTypeImpl`
classifies a struct argument or return value as direct, it leaves the
LLVM IR coerce type unspecified, implicitly relying on
`CodeGenTypes::ConvertType` to eventually construct a default IR type
based on the struct's layout. This conversion is neither stable nor
guaranteed to adhere to the ABI's classification rules.

Instead, rewrite `classifyRegCallStructTypeImpl` to construct an
explicit sequence of coerce types, using the existing field
classification to obtain a coerce type for each member of the struct.
Also, rename the function to `passRegCallStructTypeDirectly` and return
a boolean instead, so that now `classifyRegCallStructType` is the only
place that computes `ABIArgInfo`.

This rewrite also fixes several other issues with the `X86_64ABIInfo`
implementation of `__regcall`:

* Empty structs are now ignored instead of being misclassified as
direct.
* Arrays are now classified specially based on the element type, since
`X86_64ABIInfo::classifyArgumentType` ignores standalone array types.
* SSE registers used for return values are now correctly reused for
arguments, matching the 64-bit Windows behavior.

Since this is an ABI change, it has the potential to cause
incompatibilities with `__regcall` code compiled by earlier versions of
Clang. Specifically:

* Because SSE return registers can now be reused as argument registers,
functions will now pass more floating point arguments in SSE registers.
* `_Complex float` struct fields are now passed in one SSE register
instead of two.

Fixes #62999
Fixes #98635