Prevent users setting CHPL_LLVM_CONFIG to a bad value (#29036)

Adds a check to prevent users setting CHPL_LLVM_CONFIG to a bad value,
resulting in confusing errors later.

For example, if a user set `CHPL_LLVM_CONFIG=$(which llvm-config)`, they
would expect it to work. But if that llvm is too old, they could get
strange errors that do not tell them what actually went wrong (and might
look like a bug in Chapel). Or, a user could set `CHPL_LLVM_CONFIG` to
some garbage value.

Fixed by just validating `CHPL_LLVM_CONFIG` with a quick check that it
exists and is the correct version. Full validation is done at a later
step.

[Reviewed by @arifthpe]

Author: jabraham17

util/chplenv/chpl_llvm.py

@@ -279,6 +279,13 @@ def check_llvm_packages(llvm_config):
 def find_system_llvm_config():
     llvm_config = overrides.get("CHPL_LLVM_CONFIG", "none")
     if llvm_config != "none":
+        _, config_err = check_llvm_config(llvm_config)
+        if config_err:
+            error(
+                "Problem with llvm-config at {0} -- {1}".format(
+                    llvm_config, config_err
+                )
+            )
         return llvm_config
 
     llvm_config = chpl_gpu.get_llvm_override()
@@ -377,6 +384,16 @@ def get_llvm_config():
         if llvm_val == "system" or llvm_support_val == "system":
             llvm_config = find_system_llvm_config()
 
+    else:
+        # check that the provided llvm-config is valid
+        _, config_err = check_llvm_config(llvm_config)
+        if config_err:
+            error(
+                "Problem with llvm-config at {0} -- {1}".format(
+                    llvm_config, config_err
+                )
+            )
+
     return llvm_config