ini: Fix parsing of variable with spaces before the =

At least from what the source code suggests, a variable declaration in
the config file as this should be supported:
DFLAGS = -I/usr/include/d

But in reality, if you live spaces like that, the variable value will
get uppercased.

This commit fixes this problem to properly parse the extra spaces.
man: Mention variable names are converted to uppercase
ini: Add support to define undefined variables

Similarly to Make files, the syntax `VAR ?= value` is added to define
variables only when they were previously undefined.

In this case, if the environment variable `VAR` was undefined, then it
will be defined with the value `value`, otherwise the previous content
of `VAR` will be preserved.

This is useful to define default flags in the config file that can be
easily overridden by using environment variables.
Fix Issue 10221 - foreach(char c; 0..256) doesn't work even though the upper bound is 255
Move Expression::apply into a visitor class
Move lambdaInlineCost to use the apply visitor
Move lambdaSetParent and lambdaCheckForNestedRef to use the apply visitor
Move hasSideEffect to use the apply visitor
Fixes Issue 11823 - Ddoc symbol with a double underscore prefix should be specially parsed.
Merge pull request #3180 from AndrejMitrovic/Fix11823

Issue 11823 - Ddoc symbol with a double underscore prefix should be specially parsed.
fix Issue 12047 - UDAs are not checked

This is also a refactoring change for UDA semantic analysis.

1. Run UDAs in semantic2 phase.
Currently forward reference resolution mechanism is not perfect. UDAs often depends on other declarations in user code. Therefore analysing them in semantic1 phase would easily hit fwdref issues.
On the other hand, if UDAs are not checked for conditional compilation, essentially they have no effect for final execution code. So, I think moving their analysis timing from semantic1 to semantic2 is not a problem.

2. Each Dsymbols now have a pointer to the enclosing UserAttributeDeclaration.
We can use UserAttributeDeclaration::scope for forward reference resolution of UDAs. So we can elide Dsymbol::userAttributesScope.
Merge pull request #3186 from yebblies/apply

[DDMD] [refactor] Move Expression::apply into a visitor class
Remove some comments inside expressions
Remove empty statement
Explicitly compare with zero
Remove comments inside parameter lists
Move comment before switch
Move TemplateParameter comments before the class, instead of the first member
Remove commented code inside an argument list
Merge pull request #3196 from yebblies/deadcomment

[DDMD] [trivial] Remove commented code inside an argument list