codegen: Lambda improvements (#1476)

* Use #line directives in generated C++ code for lambdas

The #line directive in gcc is meant specifically for pieces of imported
code included in generated code, exactly what happens with lambdas in
the yaml files: https://gcc.gnu.org/onlinedocs/cpp/Line-Control.html

With this change, if I add the following at line 165 of kithen.yaml:
    - lambda: undefined_var == 5;

then "$ esphome kitchen.yaml compile" shows the following:

INFO Reading configuration kitchen.yaml...
INFO Generating C++ source...
INFO Compiling app...
INFO Running:  platformio run -d kitchen
<...>
Compiling .pioenvs/kitchen/src/main.cpp.o
kitchen.yaml: In lambda function:
kitchen.yaml:165:7: error: 'undefined_var' was not declared in this scope
*** [.pioenvs/kitchen/src/main.cpp.o] Error 1
== [FAILED] Took 2.37 seconds ==

* Silence gcc warning on multiline macros in lambdas

When the \ is used at the end of the C++ source in a lambda (line
continuation, often used in preprocessor macros), esphome will copy that
into main.cpp once as code and once as a // commment.  gcc will complain
about the multiline commment:

Compiling .pioenvs/kitchen/src/main.cpp.o
kitchen.yaml:640:3: warning: multi-line comment [-Wcomment]

Try to replace the \ with a "<cont>" for lack of a better idea.

esphome/cpp_generator.py

@@ -1,6 +1,7 @@
 import abc
 import inspect
 import math
+import re
 
 # pylint: disable=unused-import, wrong-import-order
 from typing import Any, Generator, List, Optional, Tuple, Type, Union, Sequence
@@ -188,13 +189,14 @@ class ParameterListExpression(Expression):
 
 
 class LambdaExpression(Expression):
-    __slots__ = ("parts", "parameters", "capture", "return_type")
+    __slots__ = ("parts", "parameters", "capture", "return_type", "source")
 
-    def __init__(self, parts, parameters, capture: str = '=', return_type=None):
+    def __init__(self, parts, parameters, capture: str = '=', return_type=None, source=None):
         self.parts = parts
         if not isinstance(parameters, ParameterListExpression):
             parameters = ParameterListExpression(*parameters)
         self.parameters = parameters
+        self.source = source
         self.capture = capture
         self.return_type = safe_exp(return_type) if return_type is not None else None
 
@@ -202,7 +204,10 @@ class LambdaExpression(Expression):
         cpp = f'[{self.capture}]({self.parameters})'
         if self.return_type is not None:
             cpp += f' -> {self.return_type}'
-        cpp += f' {{\n{self.content}\n}}'
+        cpp += ' {\n'
+        if self.source is not None:
+            cpp += f'{self.source.as_line_directive}\n'
+        cpp += f'{self.content}\n}}'
         return indent_all_but_first_and_last(cpp)
 
     @property
@@ -360,7 +365,7 @@ class LineComment(Statement):
         self.value = value
 
     def __str__(self):
-        parts = self.value.split('\n')
+        parts = re.sub(r'\\\s*\n', r'<cont>\n', self.value, re.MULTILINE).split('\n')
         parts = [f'// {x}' for x in parts]
         return '\n'.join(parts)
 
@@ -555,7 +560,7 @@ def process_lambda(
         else:
             parts[i * 3 + 1] = var
         parts[i * 3 + 2] = ''
-    yield LambdaExpression(parts, parameters, capture, return_type)
+    yield LambdaExpression(parts, parameters, capture, return_type, value.source_location)
 
 
 def is_template(value):