[LLD][COFF] Add load config checks to warn if incorrect for CFGuard

Control Flow Guard requires specific flags and VA's be included in the
load config directory to be functional. In case CFGuard is enabled via
linker flags, we can check to make sure this is the case and give the
user a warning if otherwise.

MSVC provides a proper `_load_config_used` by default, so this is more
relevant for the MinGW target in which current versions of mingw-w64
does not provide this symbol.

The checks (only if CFGuard is enabled) include:

- The `_load_config_used` struct shall exist.
- Alignment of the `_load_config_used` struct (shall be aligned to
  pointer size.)
- The `_load_config_used` struct shall be large enough to contain the
  required fields.
- The values of the following fields are checked against the expected
  values:
  - GuardCFFunctionTable
  - GuardCFFunctionCount
  - GuardFlags
  - GuardAddressTakenIatEntryTable
  - GuardAddressTakenIatEntryCount
  - GuardLongJumpTargetTable
  - GuardLongJumpTargetCount
  - GuardEHContinuationTable
  - GuardEHContinuationCount

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D133099