[clang-cl] Accept `#pragma warning(disable : N)` for some N

clang-cl maps /wdNNNN to -Wno-flags for a few warnings that map
cleanly from cl.exe concepts to clang concepts.

This patch adds support for the same numbers to
`#pragma warning(disable : NNNN)`. It also lets
`#pragma warning(push)` and `#pragma warning(pop)` have an effect,
since these are used together with `warning(disable)`.

The optional numeric argument to `warning(push)` is ignored,
as are the other non-`disable` `pragma warning()` arguments.
(Supporting `error` would be easy, but we also don't support
`/we`, and those should probably be added together.)

The motivating example is that a bunch of code (including in LLVM)
uses this idiom to locally disable warnings about calls to deprecated
functions in Windows-only code, and 4996 maps nicely to
-Wno-deprecated-declarations:

    #pragma warning(push)
    #pragma warning(disable: 4996)
      f();
    #pragma warning(pop)

Implementation-wise:
- Move `/wd` flag handling from Options.td to actual Driver-level code
- Extract the function mapping cl.exe IDs to warning groups to the
  new file clang/lib/Basic/CLWarnings.cpp
- Create a diag::Group enum so that CLWarnings.cpp can refer to
  existing groups by ID (and give DllexportExplicitInstantiationDecl
  a named group), and add a function to map a diag::Group to the
  spelling of it's associated commandline flag
- Call that new function from PragmaWarningHandler

Differential Revision: https://reviews.llvm.org/D110668