BaseTools: Honor DSC conditional branches during post-process

[kirkchoutw]

Description

When DSC conditional directives (!if/!ifdef/!ifndef/!else/!elseif) disable a branch, the data records inside that branch should not be processed during post-process. Previously, macro expansion, PCD processing, and "!include" evaluation could still be performed on records belonging to disabled branches, which could trigger build failures due to undefined macros or non-existent include files.

This PR introduces a centralized "_Enabled" gate in "DscParser._PostProcess()". Only conditional-directive records and section context records (section/subsection headers) are always processed, while all other records are skipped when the current branch is disabled. This preserves directive-stack correctness while preventing side effects from inactive
branches. "_Enabled" is also reset at the beginning of post-process to avoid stale state carried over from raw parsing.

The per-processor "if not self._Enabled: return" guards (in "_ProcessError", "__ProcessDefine", "__ProcessDirective"/include handling, and "__ProcessPcd") are removed in favor of the single gate, simplifying the logic.

• Breaking change?
  • Breaking change - Does this PR cause a break in build or boot behavior?
  • Examples: Does it add a new library class or move a module to a different repo.
  • If checked, follow the Breaking Change and Release Process.
• Impacts security?
  • Security - Does this PR have a direct security impact?
  • Examples: Crypto algorithm change or buffer overflow fix.
• Includes tests?
  • Tests - Does this PR include any explicit test code?
  • Examples: Unit tests or integration tests.

How This Was Tested

Tested with the standard build command using a real package - MdePkg.
A disabled conditional branch containing an undefined macro was temporarily added to MdePkg/MdePkg.dsc :
[LibraryClasses]
SafeIntLib|MdePkg/Library/BaseSafeIntLib/BaseSafeIntLib.inf
!if 0
DisabledLib|$(THIS_MACRO_IS_NOT_DEFINED)/DisabledLib.inf
!endif

Build command :

build -p MdePkg/MdePkg.dsc -m MdePkg/Library/BaseLib/BaseLib.inf -a IA32 -t GCC -b NOOPT

Results:

• With this fix: build completes successfully. The record inside the disabled !if 0 branch is skipped, so its undefined macro is never expanded.
• Without this fix (parser reverted to the parent commit): build aborts during DSC parsing with "MdePkg.dsc(line): error 3000: THIS_MACRO_IS_NOT_DEFINED not defined", confirming the regression this change fixes.

The same module also builds cleanly with no disabled branch present, confirming active-branch parsing is unaffected.

Integration Instructions

N/A