Improve LLVM annotations translation to SPIR-V (#1191)

https://reviews.llvm.org/D88645 introduces additional parameter
to var/ptr/global annotation instruction, which can be set by
clang::annotation attribute that has variadic constant expression
argument. Right now we can't translate it properly and an extension
is being discussed, but yet we need a W/A for this. As a W/A the
current patch makes this argument (in case if it is const int) be
translatable as a string put in UserSemantic decorations, so the
IL flow is looking like this:
Annotation in source-code:
[[clang::annotate("custom", 30, 60)]] int *Var;

LLVM IR before:
 @.str = private unnamed_addr constant [7 x i8] c"custom\00"
 @.args = private unnamed_addr constant { i32, i32 } { i32 30, i32 60 }
 // ...
 call void @llvm.var.annotation(i8* ...,
       i8* getelementptr inbounds ([7 x i8], [7 x i8]* @.str, i32 0, i32 0),
       i8* ... , i32 ..., i8* bitcast ({ i32, i32 }* @.args to i8*))
 // ...

SPIR-V:
  Decorate %GEP% UserSemantic "custom: 30, 60"

LLVM IR after:
  @3 = private unnamed_addr constant [15 x i8] c"custom: 30, 60\00"
  // ...
  call void @llvm.var.annotation(i8* ..., i8* getelementptr inbounds (
      [15 x i8], [15 x i8]* @3, i32 0, i32 0), i8* ..., i32 ..., i8* undef)
  // ...

As you see the annotation remains to be a string after the translation. It's
done so because we wouldn't be able to distinguish
[[clang::annotate("custom", 30, 60)]] from [[clang::annotate("custom, 30, 60")]]
as they both would result in the same UserSemantic decoration.

Also this patch brings a bit of refactoring of add Intel FPGA decoration functions,
since their infrustructure can (and should) be reused here, solving an issue of
incorrect allowed FPGA memory SPIR-V extensions processing.

TODO:
1. Replace this W/A with an extension, when it's ready;
2. Global annotation instruction isn't handled properly yet, its handling requires even
   more refactoring, which I plan to do, when the extension is ready.

Signed-off-by: Dmitry Sidorov <[email protected]>

Author: MrSidims