CMake: make the regexes used for setting HOST_LINK_VERSION more forgiving (PR21268)

If the output of 'ld -v' didn't match the regexes, CMake would previously error
with a message like:

  CMake Error at tools/clang/CMakeLists.txt:269 (string):
    string sub-command REGEX, mode REPLACE regex "[^0-9]*([0-9.]*).*" matched
    an empty string.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223106 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -265,7 +265,7 @@ if (APPLE)
   if (NOT HAD_ERROR)
     if ("${LD_V_OUTPUT}" MATCHES ".*ld64.*")
       string(REGEX REPLACE ".*ld64-([0-9.]*).*" "\\1" HOST_LINK_VERSION ${LD_V_OUTPUT})
-    else()
+    elseif ("${LD_V_OUTPUT}" MATCHES "[^0-9]*([0-9.]*).*")
       string(REGEX REPLACE "[^0-9]*([0-9.]*).*" "\\1" HOST_LINK_VERSION ${LD_V_OUTPUT})
     endif()
   else()