[HLSL] Implement explicit layout for $Globals

[hekota]

All non-resource variable declarations in the global scope are implicitly added to $Global constant buffer.
The declarations can have an explicit layout specifies by using the : register(c#) annotation. This task is to track implementation of the explicit layout of $Globals constant buffer.

https://godbolt.org/z/3PvocPr6v

For this global constants:

int a;
float b : register(c1);

The buffer layout should look like this:

cbuffer $Globals
{
  struct $Globals
  {
     int a;                         ; Offset:   20
     float b;                       ; Offset:   16
  } $Globals;                       ; Offset:    0 Size:    24
}

The declarations without register(c#) annotations are placed in the buffer after the explicit ones.

[damyanp]

This is blocked on $Globals support being completed.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Helena Kotas (hekota)

All non-resource variable declarations in the global scope are implicitly added to `$Global` constant buffer. The declarations can have an explicit layout specifies by using the `: register(c#)` annotation. This task is to track implementation of the explicit layout of `$Globals` constant buffer.

https://godbolt.org/z/3PvocPr6v

For this global constants:

int a;
float b : register(c1);

The buffer layout should look like this:

cbuffer $Globals
{
  struct $Globals
  {
     int a;                         ; Offset:   20
     float b;                       ; Offset:   16
  } $Globals;                       ; Offset:    0 Size:    24
}

The declarations without register(c#) annotations are placed in the buffer after the explicit ones.