llvm: make bottle compatible with Xcode-only installs

[carlocab]

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---

This patch implements various improvements I found while working on #77975.

A few are cosmetic, but the primary substantive change is the use of the
C_INCLUDE_DIRS CMake variable. [1]

This adds Homebrew's include directory along with Xcode's system
header path to Clang's default include search path. The former change
aligns our Clang with Apple's, which searches /usr/local/include by
default. [2] The latter change allows the bottle to be poured in
Xcode-only installs so that we no longer need the pour_bottle? check.

We also add /Library/Developer/CommandLineTools/usr/include to the
default header search path to align our build with Apple's.

[1] https://reviews.llvm.org/D69221
[2] From /usr/bin/clang -E -xc -v /dev/null:

Apple clang version 12.0.5 (clang-1205.0.22.9)
[snip]
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /Library/Developer/CommandLineTools/usr/lib/clang/12.0.5/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
End of search list.
[snip]

[carlocab]

For reference, with our bottled LLVM:

❯ ~/homebrew/opt/llvm/bin/clang -E -xc -v /dev/null
clang version 12.0.0
[snip]
#include "..." search starts here:
#include <...> search starts here:
 /Users/carlocab/homebrew/Cellar/llvm/12.0.0_1/lib/clang/12.0.0/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
End of search list.

[carlocab]

I'm hoping this won't affect the bottle being cellar: :any, since this is stored in a text file. If it makes the bottle non-relocatable we can drop it.

[Bo98]

C_INCLUDE_DIRS should be baked in to clang binaries so it will affect relocatability.

[carlocab]

DEFAULT_SYSROOT isn't (at least, not in a way you can find with strings), so I was maybe a bit hopeful here.

[carlocab]

Killing the tests while #79465 is running.

[danielnachun]

Sorry to make you wait to test this! I'll wait until you've had a chance to run this before I push any more changes to the LLVM PRs I have open.

[carlocab]

Oh, wait, this might not be a good idea. https://github.com/Homebrew/linuxbrew-core/pull/22742#issuecomment-810641119

Will need to look into this some more.

[Bo98]

The latter change allows the bottle to be poured in
Xcode-only installs so that we no longer need the pour_bottle? check.

C_INCLUDE_DIRS behaves differently to DEFAULT_SYSROOT. The latter can be overridden with -isysroot. The former only can with the combination of -nostdlibinc & -isystem. This is a significant change of behaviour for those who want to use a specified SDK.

I also think C++ stdlib header handling won't read C_INCLUDE_DIRS but will read DEFAULT_SYSROOT.

[Bo98]

Apple's way of handling CLT & Xcode does not lie within LLVM itself - it's with how /usr/bin/clang goes through xcrun, which sets the SDKROOT env.

[carlocab]

Yup, realising that now. Though, xcrun seems to be doing more than just setting SDKROOT, since Apple Clang is also including /Library/Developer/CommandLineTools/usr/include.