Enable C language in CMakeList to avoid error finding LLVM

With the absortion of
swiftlang/llvm-project@b663395
in stable/20220421, the LLVMConfig might try to execute C code if
Terminfo library is found in the system. However the project line of
XCTest does not enable the C language, and finding the package might
fail in that case.

This is not a problem in Swift CI because the CI machines does not seem
to have the Terminfo library (as reported in their logs during the
configuration of XCTest). This might only affect developers in their
local machines.

Author: drodriguez

CMakeLists.txt

@@ -69,6 +69,7 @@ set_target_properties(XCTest PROPERTIES
 if(ENABLE_TESTING)
   enable_testing()
 
+  enable_language(C)
   find_package(LLVM CONFIG)
   if(LLVM_FOUND)
     message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")