Define `LLVM` symbol on LLVM-based toolchains from Delphi 12 onward

[cirras]

Prerequisites

• This improvement has not already been suggested.
• This improvement would be generally useful, not specific to my code or setup.

Engine area

Delphi language support

Improvement description

LLVM should be treated as defined on LLVM-based toolchains.

See:

• 3 x 12 IDE, Installer and Delphi Language Enhancements in Delphi 12
• Delphi Toolchains

Rationale

Delphi 12 support.