This had a few tricky areas to resolve:
- the AMD method of querying many extensions for every single
   token was fixed (big perf change in scanning) in GitHub, but
   expanded upon in GitLab, so the new GitLab changes had to be
   refactored to match the GitHub way
- two competing implementations of SPV target versioning, favored
   the GitHub version

Add DeadInsertElim to legalization and RedundancyElim to -Os.

SPV: Create more access chains addressing a few swizzling issues.

Preprocessor: Use std::string instead of std::stringstream

- Fixes #1233
- Treats local bools like anything else
- more consistently deals with a dynamic component selection

std::stringstream has a measurable overhead for preprocessing - it
appears that operator<< does a tiny bit of extra work for appending
chars/strings and also can't be inlined in most cases on VS2015;
additionally, std::endl triggers a stream flush which also adds up.

Replacing this with std::string buffer gets the preprocessing time down
from 180ms to 135ms in one case, making it 1.33x faster.

Note that integer-to-string conversion is using std::to_string; in
theory this could be slower than sprintf or manual conversion, but I
haven't found these cases to affect preprocessing time in practice
(std::to_string would always use the short string buffer for
line/version numbers, and the number of calls is not too significant).

Update SPIRV-Tools known-good

Update SPIRV-Tools.  Relevant functional changes:
 - Optimizer enhancements:
   - ADCE now removes OpSwitch
   - Block merging occurs in more cases
 - Optimizer fixes:
   - Constant propagation (CCP): support matrix constants
   - #1199: Optimizer: Fix CCP: don't propagate spec constants.
   - #1203: Optimizer: Fix common uniform elim bug introduced by refactoring.
   - #1210: Optimizer: Aggressive dead code elimination: Fix 'break' identification.
   - #1212: Optimizer: Aggressive dead code elimination: Was skipping too many instructions.
   - #1214: Optimizer: Aggressive dead code elimination: Fix infinite loop.
   - #1228: Optimizer: Fix CCP: Handling of varying Phi nodes; was resulting in infinite loop.
   - #1245: Optimizer: Dead branch elimination: Avoid a null pointer dereference.
   - #1250: Optimizer: Dead branch elimination: Avoid spuriously reporting a change.
   - #1262: Support building on VisualStudio 2013 again

Update SPIRV-Headers, with "unified1" directory.

Updated one Glslang legalization test base result due to better block merging.

Control flow attributes

See https://github.com/KhronosGroup/GLSL/pull/11.

- make it sharable with GLSL
- correct the case insensitivity
- remove the map; queries are not needed, all entries need processing
- make it easier to build bottom up (will help GLSL parsing)
- support semantic checking and reporting
- allow front-end dependent semantics and attribute name mapping

Fixes #1236.

n why this merge is necessary,

MSVC annoying totally redid their wave ops here:

https://github.com/Microsoft/DirectXShaderCompiler/wiki/Wave-Intrinsics

I've had to totally rework the existing wave ops to support this.

```
[----------] Global test environment tear-down
[==========] 1142 tests from 44 test cases ran. (186552 ms total)
[  PASSED  ] 1142 tests.
```

Fix unused function warning.

Update known-good spirv-tools.

TGlslangToSpvTraverser::getExtBuiltins is only used when AMD_EXTENSIONS
is defined, so only define it in that case to avoid an unused function
warning.

Optimizations used in HLSL legalization now sweep away unused types.

- correct inheritence (or not) of the right XFB buffer
- compute implicit stride (fixes #1212)
- semantic check block-member redeclarations
- inherit stride from a member

- fixes #1209, addresses most of #1187
- only query feature availability on seeing the feature
  (was doing it for every single token)
- correct case-sensitive checks for multi-character suffixes

- partially addresses #1209 and #1187
- only query 64-bit extension on seeing use of a 64-bit literal
  (was doing it for every single token)
- correct HLSL acceptance of 64-bit literal syntax (still an int though)
- error on overflow of 32-bit literal type

Update known good spirv-tools. Fixes issue #1205.