Fix issue where MACOSX_VERSION_MIN_FLAG was not set on subsequent runs of CMake in compiler-rt

[cjappl]

As discussed here:

#74394 (comment)

An unintentional change of behavior was introduced in #74394

This code introduced in #74394 :

The first time through

• SANITIZER_MIN_OSX_VERSION is not set
• parse -mmacosx-version-min and set MACOSX_VERSION_MIN_FLAG
• Set and cache SANITIZER_MIN_OSX_VERSION

Subsequent times through:

• SANITIZER_MIN_OSX_VERSION is cached
• (BUG!!) you don't parse -mmacosx-version-min, and don't set MACOSX_VERSION_MIN_FLAG

MACOSX_VERSION_MIN_FLAG is used later in the file on this line:

llvm-project/compiler-rt/cmake/config-ix.cmake

Line 517 in 63c925c

if(NOT MACOSX_VERSION_MIN_FLAG)

Hoisting this assignment outside the if block returns us to the previous behavior before this commit, while maintaining the flexibility introduced with the cache variable

[github-actions]

⚠️ We detected that you are using a GitHub private e-mail address to contribute to the repo.
Please turn off Keep my email addresses private setting in your account.
See LLVM Discourse for more information.

[cjappl]

CC for a review:

@petrhosek @yln @vitalybuka

[yln]

Thank you!