Add support for user-provided hooks and environment variable reads to the cmd_line tool property.

Used like this: (cmd_line "$CALL(MyHook) --option -o $ENV(VARIABLE) $CALL(AnotherHook)")
Also works with case expressions.

Hook declarations are auto-generated, the definitions should be provided by the user
(just drop a .cpp file in the tools/llvmc2 directory).

Hooks should live in the "hooks" namespace and have type std::string hooks::Hook(void).

llvm-svn: 51732