Control Werror by converting to Wno-error (#30882)

Using `-Werror` is good practice for development and testing, but causes us a great
deal of heartburn supporting multiple compiler versions, especially as newer compiler
versions add warnings for released packages.  This PR adds support for suppressing
`-Werror` through spack's compiler wrappers.  There are currently three modes for
the `flags:keep_werror` setting:

* `none`: (default) cancel all `-Werror`, `-Werror=*` and `-Werror-*` flags by
  converting them to `-Wno-error[=]*` flags
* `specific`: preserve explicitly selected warnings as errors, such as
  `-Werror=format-truncation`, but reverse the blanket `-Werror`
* `all`: keeps all `-Werror` flags

These can be set globally in config.yaml, through the config command-line flags, or
overridden by a particular package (some packages use Werror as a proxy for determining
support for other compiler features).  We chose to use this approach because:

1. removing `-Werror` flags entirely broke *many* build systems, especially autoconf
   based ones, because of things like checking `-Werror=feature` and making the
   assumption that if that did not error other flags related to that feature would also work
2. Attempting to preserve `-Werror` in some phases but not others caused similar issues
3. The per-package setting came about because some packages, even with all these
   protections, still use `-Werror` unsafely.  Currently there are roughly 3 such packages
   known.

lib/spack/env/cc

@@ -440,6 +440,47 @@ while [ $# -ne 0 ]; do
         continue
     fi
 
+    if [ -n "${SPACK_COMPILER_FLAGS_KEEP}" ] ; then
+        # NOTE: the eval is required to allow `|` alternatives inside the variable
+        eval "\
+        case \"\$1\" in
+            $SPACK_COMPILER_FLAGS_KEEP)
+                append other_args_list \"\$1\"
+                shift
+                continue
+                ;;
+        esac
+        "
+    fi
+    # the replace list is a space-separated list of pipe-separated pairs,
+    # the first in each pair is the original prefix to be matched, the
+    # second is the replacement prefix
+    if [ -n "${SPACK_COMPILER_FLAGS_REPLACE}" ] ; then
+        for rep in ${SPACK_COMPILER_FLAGS_REPLACE} ; do
+            before=${rep%|*}
+            after=${rep#*|}
+            eval "\
+            stripped=\"\${1##$before}\"
+            "
+            if [ "$stripped" = "$1" ] ; then
+                continue
+            fi
+
+            replaced="$after$stripped" 
+
+            # it matched, remove it
+            shift
+
+            if [ -z "$replaced" ] ; then
+                # completely removed, continue OUTER loop
+                continue 2
+            fi
+
+            # re-build argument list with replacement
+            set -- "$replaced" "$@"
+        done
+    fi
+
     case "$1" in
         -isystem*)
             arg="${1#-isystem}"