[compiler-rt] Fix running tests on macOS when XCode is not installed
[compiler-rt] Fix running tests on macOS when XCode is not installed

Summary:
If XCode is not installed, `xcodebuild -version -sdk macosx Path` will give
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance
In this case the variable OSX_SYSROOT will be empty and
OSX_SYSROOT_FLAG is set to "-isysroot" (without a path).
This then causes the CompilerRTUnitTestCheckCxx target failed to for me
because "${COMPILER_RT_TEST_COMPILER} ${OSX_SYSROOT_FLAG} -E" expanded to
"clang -isysroot -E". This results in a warning "sysroot -E does not exist"
and the target fails to run because the C++ headers cannot be found.

Reviewers: beanz, kubamracek

Reviewed By: beanz

Subscribers: dberris, mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D65323

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@367170 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt