[MS] Overhaul how clang passes overaligned args on x86_32

MSVC 2013 would refuse to pass highly aligned things (typically vectors
and aggregates) by value. Users would receive this error:
  t.cpp(11) : error C2719: 'w': formal parameter with __declspec(align('32')) won't be aligned
  t.cpp(11) : error C2719: 'q': formal parameter with __declspec(align('32')) won't be aligned

However, in MSVC 2015, this behavior was changed, and highly aligned
things are now passed indirectly. To avoid breaking backwards
incompatibility, objects that do not have a *required* high alignment
(i.e. double) are still passed directly, even though they are not
naturally aligned. This change implements the new behavior of passing
things indirectly.

The new behavior is:
- up to three vector parameters can be passed in [XYZ]MM0-2
- remaining arguments with required alignment greater than 4 bytes are
  passed indirectly

Previously, MSVC never passed things truly indirectly, meaning clang
would always apply the byval attribute to indirect arguments. We had to
go to the trouble of adding inalloca so that non-trivially copyable C++
types could be passed in place without copying the object
representation. When inalloca was added, we asserted that all arguments
passed indirectly must use byval. With this change, that assert no
longer holds, and I had to update inalloca to handle that case. The
implicit sret pointer parameter was already handled this way, and this
change generalizes some of that logic to arguments.

There are two cases that this change leaves unfixed:
1. objects that are non-trivially copyable *and* overaligned
2. vectorcall + inalloca + vectors

For case 1, I need to touch C++ ABI code in MicrosoftCXXABI.cpp, so I
want to do it in a follow-up.

For case 2, my fix is one line, but it will require updating IR tests to
use lots of inreg, so I wanted to separate it out.

Related to D71915 and D72110

Fixes most of PR44395

Reviewed By: rjmccall, craig.topper, erichkeane

Differential Revision: https://reviews.llvm.org/D72114

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