Print printchplenv warnings at the end of the output

[jabraham17]

Changes how printchplenv prints warnings so that the warnings are outputed last.

Prior to this PR, printchplenv warnings could get lost if the terminal scrolled past. This PR puts the warnings last so that users can be more directly notified. The exception is in the error case. If there was an error and printchplenv is going to exit, all to the warnings are printed before the errors, so that users can see warnings that may have lead to the error being produced.

Note: if a user throws --ignore-errors, no warning buffering is done, otherwise printchplenv may crash without dumping the buffer out.

Testing:

• CHPL_LLVM_VERSION=2 printchplenv --all shows the warning at the bottom
• CHPL_DEVELOPER=1 CHPL_TARGET_COMPILER=gnu CHPL_TARGET_CC=clang CHPL_LLVM_VERSION=2 printchplenv --all shows the warning and then the exception
• CHPL_DEVELOPER=0 CHPL_TARGET_COMPILER=gnu CHPL_TARGET_CC=clang CHPL_LLVM_VERSION=2 printchplenv --all shows the warning and then prints the error
• CHPL_TARGET_COMPILER=gnu CHPL_TARGET_CC=clang CHPL_LLVM_VERSION=2 printchplenv --all --ignore-errors does no buffering

[Reviewed by @vasslitvinov]

[vasslitvinov]

Great, thanks!