[clang-cl] Support the /JMC flag

The introduction and some examples are on this page:
https://devblogs.microsoft.com/cppblog/announcing-jmc-stepping-in-visual-studio/

The `/JMC` flag enables these instrumentations:
- Insert at the beginning of every function immediately after the prologue with
  a call to `void __fastcall __CheckForDebuggerJustMyCode(unsigned char *JMC_flag)`.
  The argument for `__CheckForDebuggerJustMyCode` is the address of a boolean
  global variable (the global variable is initialized to 1) with the name
  convention `__<hash>_<filename>`. All such global variables are placed in
  the `.msvcjmc` section.
- The `<hash>` part of `__<hash>_<filename>` has a one-to-one mapping
  with a directory path. MSVC uses some unknown hashing function. Here I
  used DJB.
- Add a dummy/empty COMDAT function `__JustMyCode_Default`.
- Add `/alternatename:__CheckForDebuggerJustMyCode=__JustMyCode_Default` link
  option via ".drectve" section. This is to prevent failure in
  case `__CheckForDebuggerJustMyCode` is not provided during linking.

Implementation:
All the instrumentations are implemented in an IR codegen pass. The pass is placed immediately before CodeGenPrepare pass. This is to not interfere with mid-end optimizations and make the instrumentation target-independent (I'm still working on an ELF port in a separate patch).

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D118428