[CMake][XRay] Silence llvm-config error when checking library support

[phosek]

Summary

Otherwise this propagates all the way to CMake and results in an error
during configuration. We check and handle the result and report warning
separately so this is not changing the behavior.

[dberris]

LGTM

compiler-rt/trunk/cmake/Modules/CompilerRTUtils.cmake

if (LLVM_CONFIG_PATH)
     set(LLVM_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
     set(LLVM_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Paths to LLVM headers")
 
     # Detect if we have the LLVMXRay and TestingSupport library installed and
     # available from llvm-config.
     execute_process(
       COMMAND ${LLVM_CONFIG_PATH} "--ldflags" "--libs" "xray"
       RESULT_VARIABLE HAD_ERROR
-      OUTPUT_VARIABLE CONFIG_OUTPUT)
+      OUTPUT_VARIABLE CONFIG_OUTPUT
+      ERROR_QUIET)
     if (HAD_ERROR)
       message(WARNING "llvm-config finding xray failed with status ${HAD_ERROR}")
       set(COMPILER_RT_HAS_LLVMXRAY FALSE)
     else()
       string(REGEX REPLACE "[ \t]*[\r\n]+[ \t]*" ";" CONFIG_OUTPUT ${CONFIG_OUTPUT})
       list(GET CONFIG_OUTPUT 0 LDFLAGS)
       list(GET CONFIG_OUTPUT 1 LIBLIST)
       set(LLVM_XRAY_LDFLAGS ${LDFLAGS} CACHE STRING "Linker flags for LLVMXRay library")
       set(LLVM_XRAY_LIBLIST ${LIBLIST} CACHE STRING "Library list for LLVMXRay")
       set(COMPILER_RT_HAS_LLVMXRAY TRUE)
     endif()
 
     set(COMPILER_RT_HAS_LLVMTESTINGSUPPORT FALSE)
     execute_process(
       COMMAND ${LLVM_CONFIG_PATH} "--ldflags" "--libs" "testingsupport"
       RESULT_VARIABLE HAD_ERROR
-      OUTPUT_VARIABLE CONFIG_OUTPUT)
+      OUTPUT_VARIABLE CONFIG_OUTPUT
+      ERROR_QUIET)
     if (HAD_ERROR)
       message(WARNING "llvm-config finding testingsupport failed with status ${HAD_ERROR}")
     else()
       string(REGEX REPLACE "[ \t]*[\r\n]+[ \t]*" ";" CONFIG_OUTPUT ${CONFIG_OUTPUT})
       list(GET CONFIG_OUTPUT 0 LDFLAGS)
       list(GET CONFIG_OUTPUT 1 LIBLIST)
       if (LIBLIST STREQUAL "")
         message(WARNING "testingsupport library not installed, some tests will be skipped")