[clang] [profile] Add a clang option -fprofile-continuous that enables continuous instrumentation profiling mode (PR #124353)

[Wael Yehia via cfe-commits]

w2yehia wrote:

> > Just saw this in LLVM weekly. But this is not supported on Windows right? Only the UNIX platforms that can mmap? In that case it the limitation needs to be documented
> 
> It is supported! We have [an `mmap` implementation for Windows](https://github.com/llvm/llvm-project/blob/d7fd2a2a3befe451d3cc70612a40e8c90a780caf/compiler-rt/lib/profile/WindowsMMap.c) in the profile runtime and [the continuous mode is being tested in Windows](https://github.com/llvm/llvm-project/blob/d7fd2a2a3befe451d3cc70612a40e8c90a780caf/compiler-rt/test/profile/ContinuousSyncMode/runtime-counter-relocation.c).

Thanks Petr. The Source Coverage page has:
> Support for Linux may be mostly complete but requires testing, and support for Windows may require more extensive changes: please get involved if you are interested in porting this feature.

Should we keep it as is or improve it?

https://github.com/llvm/llvm-project/pull/124353