feat: add `drop constraint if exists` to compiler

### Description

Add `DROP CONSTRAINT ... IF EXISTS` behavior to the compiler.

Fixes https://github.com/sqlalchemy/sqlalchemy/issues/8141

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [x] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #8161
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8161
Pull-request-sha: 43276e29fa864fc66900c5a3fa0bf84df5f14271

Change-Id: I18bae3cf013159b6fffde4413fb59ce19ff83c16

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 60ec09771..817fc9315 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -5216,10 +5216,11 @@ class DDLCompiler(Compiled):
                 "Can't emit DROP CONSTRAINT for constraint %r; "
                 "it has no name" % drop.element
             )
-        return "ALTER TABLE %s DROP CONSTRAINT %s%s" % (
+        return "ALTER TABLE %s DROP CONSTRAINT %s%s%s" % (
             self.preparer.format_table(drop.element.table),
+            "IF EXISTS " if drop.if_exists else "",
             formatted_name,
-            drop.cascade and " CASCADE" or "",
+            " CASCADE" if drop.cascade else "",
         )
 
     def get_column_specification(self, column, **kwargs):