Fold LLVM-or-not into CHPL_TARGET_COMPILER

[mppf]

For #17795

Resolves #17606 #17598 #17584

This PR updates printchplenv, Makefiles, and related scripting to:

• When CHPL_LLVM=system or CHPL_LLVM=bundled, CHPL_TARGET_COMPILER
  defaults to llvm (per Should LLVM-based compiles be indicated through CHPL_TARGET_COMPILER rather than --llvm? #17795 )
• have an idea of a prefix and suffix for the current compiler (e.g.
  CHPL_GCC_PREFIX=/usr/local/ and CHPL_GCC_SUFFIX=-10 would select
  /usr/local/bin/gcc-10 and /usr/local/bin/g++-10)
• Along with that, adds CHPL_LLVM_PREFIX for selecting an installation
  of LLVM
• adds a warning when doing an LLVM-enabled make of the compiler with
  a PrgEnv and when that PrgEnv is not PrgEnv-gnu. This is just to get
  people to use configurations that we test.
• Replaces CHPL_TARGET_COMPILER=clang-included with
  CHPL_TARGET_COMPILER=llvm
• Adds internal printchplenv variables CHPL_HOST_COMPILER_COMMAND_C
CHPL_HOST_COMPILER_COMMAND_CPP CHPL_TARGET_COMPILER_COMMAND_C
CHPL_TARGET_COMPILER_COMMAND_CPP and uses them in the Makefiles
  • This resolves a problem I was seeing with PrgEnv-cray where the
    compiler was trying to run clang instead of cc
• Stops building the runtime twice for LLVM per Should LLVM-based compiles be indicated through CHPL_TARGET_COMPILER rather than --llvm? #17795
• removes CHPL_ORIG_TARGET_COMPILER (replacing it with the more
  focused CHPL_TARGET_COMPILER_PRGENV which just indicates which
  PrgEnv is loaded, if any)
• Renames a few Makefile helpers in third-party/llvm to use the term
  "bundled"
• Moves setting LLVM_CLANG_ARGUMENTS_FILE to only CHPL_LLVM=bundled
  configurations since it should only be relevant in that case
• many supporting changes in the util/chplenv scripts

Future Work:

• Update CHPL_LLVM_PREFIX and the commented-out CHPL_GCC_PREFIX /
  CHPL_GCC_SUFFIX / CHPL_CLANG_PREFIX / CHPL_CLANG_SUFFIX with
  whatever we decide in that discussion
• Consider making CHPL_TARGET_COMPILER=clang + CHPL_LLVM=bundled an
  error in a PrgEnv environment since it doesn't work right now (we
  don't gather the appropriate extra flags for e.g. ugni dependencies)

• full local testing with CHPL_LLVM=system
• full local testing with CHPL_LLVM=none

Reviewed by @daviditen - thanks!

[daviditen]

Should the default base compiler be clang instead of gcc?

[mppf]

I'm not expecting this to ever get run - we could make it a fatal error instead if we wanted. I'm not sure there really is a good option but I'd bet linux64 is slightly more typical of a platform for using Chapel than Mac, but who knows.

Edit -- also on Mac OS X there is a gcc that just runs clang -- but I wouldn't want to rely on that for much.