mformat: A triple string with a ' in it cannot be simplified

The following is valid meson:
```meson
a = '''This string can't be simplified'''
```
which cannot be simplified because of the `'` in it, as
```meson
a = 'This string can't be simplified'
```
Is invalid.

Potentially we could convert that with escapes, but it seems reasonable
to me to leave this, since it may be desirable to not have lots of
escapes in a string. `'''I can't believe it's her's!'''` is much more
readable than `'I can\'t believe it\'s her\'s!'`, for example.

Closes: #13564

mesonbuild/mformat.py

@@ -338,7 +338,7 @@ class TrimWhitespaces(FullAstVisitor):
         self.enter_node(node)
 
         if self.config.simplify_string_literals:
-            if node.is_multiline and '\n' not in node.value:
+            if node.is_multiline and not any(x in node.value for x in ['\n', "'"]):
                 node.is_multiline = False
                 node.value = node.escape()
 

test cases/format/5 transform/default.expected.meson

@@ -47,6 +47,7 @@ d = {'a': 1, 'b': 2, 'c': 3}
 # string conversion
 'This is not a multiline'
 'This is not a fstring'
+'''This isn't convertible'''
 
 # group arg value
 arguments = [

test cases/format/5 transform/muon.expected.meson

@@ -47,6 +47,7 @@ d = {'a': 1, 'b': 2, 'c': 3}
 # string conversion
 '''This is not a multiline'''
 f'This is not a fstring'
+'''This isn't convertible'''
 
 # group arg value
 arguments = [

test cases/format/5 transform/options.expected.meson

@@ -29,6 +29,7 @@ d = {
 # string conversion
 'This is not a multiline'
 'This is not a fstring'
+'''This isn't convertible'''
 
 # group arg value
 arguments = [

test cases/format/5 transform/source.meson

@@ -29,6 +29,7 @@ d = {'a': 1, 'b': 2, 'c': 3}
 # string conversion
 '''This is not a multiline'''
 f'This is not a fstring'
+'''This isn't convertible'''
 
 # group arg value
 arguments = ['a', '--opt_a', 'opt_a_value', 'b', 'c', '--opt_d', '--opt_e', 'opt_e_value',