RFR: 8253402: Convert vmSymbols::SID to enum class [v5]

[Ioi Lam]

> Convert `vmSymbols::SID` to an `enum class` to provide better type safety.
> 
> - The original enum type `vmSymbols::SID` cannot be forward-declared. I moved it out of the `vmSymbols` class and
>   renamed, so now it can be forward-declared as `enum class vmSymbolID : int;`, without including the large vmSymbols.hpp
>   file.
>   - This also breaks  the mutual dependency between the `vmSymbols` and `vmIntrinsics` classes. Now  the declaration of
>     `vmIntrinsics` can be moved from vmSymbols.hpp to vmIntrinsics.hpp, where it naturally belongs.
> - Type-safe enumeration (contributed by Kim Barrett)
> for (vmSymbolsIterator it = vmSymbolsRange.begin(); it != vmSymbolsRange.end(); ++it) {
>  vmSymbolID index = *it; ....
> }
> - I moved `vmSymbols::_symbols[]` to `Symbol::_vm_symbols[]`, and made it accessible via `Symbol::vm_symbol_at()`. This
>   way, header files (e.g. fieldInfo.hpp) that need to convert from `vmSymbolID` to `Symbol*` don't need to include the
>   large vmSymbols.hpp file.
> - I changed the `VM_SYMBOL_ENUM_NAME` macro so that the users don't need to explicitly add the `vmSymbolID::` scope.
> - I removed many unnecessary casts between `int` and `vmSymbolID`.
> - The remaining casts are done via `vmSymbol::as_int()` and `vmSymbols::as_SID()` with range checks.
> 
> -----
> If this is successful, I will do the same for `vmIntrinsics::ID`.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  Addressed review comments by Kim Barrett

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/276/files
  - new: https://git.openjdk.java.net/jdk/pull/276/files/9ddca08f..662ad8b2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=276&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=276&range=03-04

  Stats: 13 lines in 2 files changed: 4 ins; 1 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/276.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/276/head:pull/276

PR: https://git.openjdk.java.net/jdk/pull/276