C++ Modules TS: support parsing the 'module' declaration (including extensions

C++ Modules TS: support parsing the 'module' declaration (including extensions
from p0273r0 approved by EWG). We'll eventually need to handle this from the
lexer as well, in order to disallow preprocessor directives preceding the
module declaration and to support macro import.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@279196 91177308-0d34-0410-b5e6-96231b3b80d8