[flang] Improve compiler_version test (NFC)

[kkwli0]

Summary

This patch is to improve the compiler_version test. Some downstream implementations may prepend a string to the compiler_version in addition to "flang version ...". Our internal build breakage is due to this issue. In addition, the check is modified to verify that "flang version" is followed by numbers in the form of "n.n.n".

[DanielCChen]

LGTM.

[kkwli0]

Remove the extra space in the string

[awarzynski]

LGTM

flang/test/Evaluate/compiler_version.f90

 ! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
 
 program main
     use ISO_FORTRAN_ENV, only: compiler_version
     implicit none
     character (len = :), allocatable :: v
-! CHECK: v="flang version {{.*}}"
+! CHECK: v="{{.*}}flang version {{[0-9]*.[0-9]*.[0-9]}} {{.*}}"
     v = compiler_version()
     print *, v
     deallocate(v)
     close(1)
 end program main