build: fix spelling of clang

[edymtt]

Addresses rdar://problem/56866783

[edymtt]

@swift-ci test

[shahmishal]

@swift-ci test Windows

[compnerd]

Sure, though this doesnt really have any functional change. The Clang spelling is correct, and should match both AppleClang and Clang as identified by CMake. Furthermore, the check is case insensitive, so this should remain the same? If I'm wrong about the case, we should notice it pretty quickly with failures on Azure.

[edymtt]

According to our tests, MATCHES seems case sensitive (and I cannot see any way to make that case sensitive).

Also, CMAKE_C_COMPILER seems to point to a path (in which case I would expect clang to be lower case) -- wondering if we should test CMAKE_C_COMPILER_ID instead

[compnerd]

UGH, you are correct, the variable being checked is incorrect - that was supposed to be CMAKE_C_COMPILER_ID, not CMAKE_C_COMPILER.

[compnerd]

I put up #28058 that corrects the variable. Sorry about that!

[edymtt]

Thanks!

Duplicate of #28058

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 6a8bb26

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 6a8bb26