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

[Petr Hosek via cfe-commits]

petrhosek 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).

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