chore: exclude underscore placeholder from shadowing linter warnings

Bitcoin uses underscore in util/translation.h for translatable strings
but underscores are also a placeholder used in multiple languages, incl.
C++. The next commit will be introducing backports, one of them will be
placing util/translation.h into validation.h, which is a header used by
Dash-specific code.

This causes a conflict between the normal usage of underscore as a
placeholder and Bitcoin's usage of it as a function name, which is
reported by the Dash-specific linter. We need to exclude shadowing
warnings from the linter to account for this.

test/lint/lint-cppcheck-dash.sh

@@ -21,7 +21,7 @@ ENABLED_CHECKS=(
     "Variable '.*' is assigned a value that is never used."
     "Unused variable"
     "The function '.*' overrides a function in a base class but is not marked with a 'override' specifier."
-# Enabale to catch all warnings
+# Enable to catch all warnings
     ".*"
 )
 
@@ -34,13 +34,14 @@ IGNORED_WARNINGS=(
     "src/rpc/.*cpp:.*: note: Function pointer used here."
     "src/masternode/sync.cpp:.*: warning: Variable 'pnode' can be declared as pointer to const \[constVariableReference\]"
     "src/wallet/bip39.cpp.*: warning: The scope of the variable 'ssCurrentWord' can be reduced. \[variableScope\]"
-
+    "src/.*:.*: warning: Local variable '_' shadows outer function \[shadowFunction\]"
 
     "src/stacktraces.cpp:.*: .*: Parameter 'info' can be declared as pointer to const"
     "src/stacktraces.cpp:.*: note: You might need to cast the function pointer here"
 
     "[note|warning]: Return value 'state.Invalid(.*)' is always false"
     "note: Calling function 'Invalid' returns 0"
+    "note: Shadow variable"
 
 # General catchall, for some reason any value named 'hash' is viewed as never used.
     "Variable 'hash' is assigned a value that is never used."