HLSL: Allow primitive id on hull shader input

HLSL: Stop flattening non-IO structs containing opaques.

Fixes #979

This makes struct returns from functions work, but breaks
structs containing arrays, due to limitations in subsequent
transforms in spirv-opt. This is expected to be fixed soon.

Improve build instructions

- Mention that the instructions are written for Bash.
- Quote the CMAKE_INSTALL_PREFIX to handle spaces in $(pwd).
- Remove SSH clone instructions. Instructions on how to clone are for
  those unfamiliar with GitHub, so fewer options are better and SSH
  requires additional configuration to make work.
- Replace ninja with make, because more people are familiar with it and
  we probably don't need to explain how to install it. Fixes #956.
- Remove the section mentioning cygwin, as it's unclear and doesn't
  seem necessary.
- Mention minimum language version in the Dependencies section.
- Move `cd $BUILD_DIR` out of the Linux configuration section because
  it's needed on both platforms. Add a line about creating the build
  directory, too.

This continues to prevent writing output buffers (out from a function),
but fixes the problem where the copy-in/out was not getting done.

Making everything work will require knowing both in/out-ness and bufferness,
but these are currently mutually exclusive, because both are storage
qualifiers.

Update spirv-tools known-good

HLSL: minor: add warning for mat() matrix size truncation

This fixes spirv-tools issue 989 where if-break can be incorrectly
deleted from a loop.

Minor change to add a compilation warning on implicit matrix size
truncations.

(This is motivated by wanting to retickle the tests.)

#1165 invalid store

HLSL: allow keyword-identifiers as cbuffer/struct names.

HLSL: add implicit mat*mat truncations

Update spirv-tools known-good

Issue #791 was partially fixed by PR #1161 (the mat mul implicit
truncations were its main point), but it still wouldn't compile due to
the use of ConstantBuffer as an identifier.  Apparently those fall into
the same class as "float float", where float is both a type and an
identifier.

This allows struct definitions with such keyword-identifiers,
and adds ConstantBuffer to the set.  'cbuffer int' is legal in HLSL,
and 'struct int' appears to only be rejected due to the redefinition
of the 'int' type.

Fixes #791

This includes an enhancement to propagate through nested structs.

Goes with PR #1161, and completes the space for mul() implicit truncations.

Note that the v*v and scalar cases are already handled by existing code.

WIP: HLSL: matrix and vector truncations for m*v, v*m, m*m

HLSL truncates the vector, or one of the two matrix dimensions if there is a
dimensional mismatch in m*v, v*m, or m*m.

This PR adds that ability.  Conversion constructors are added as required.

Completes a TODO from previous commit.

Fixes #916 (the last change covered by the PR).
3rd list item in #976.

Fixes #1158.
Fixes #1159.

Addresses 2nd item in #976.

HLSL: Accept unorm and snorm on types

HLSL: ignore geometry attributes on non-GS stages.

If a shader includes a mixture of several stages, such as HS and GS,
the non-stage output geometry should be ignored, lest it conflict
with the stage output.

Memory fixes, round 1

Appveyor: avoid double testing on master commits

This is currently parsed and ignored, save for some minor validation.