[NFC] Address compiler warnings: C4146 - A 'grab bag' of remaining instances

[alsepkow]

Fix C4146 warnings: unary minus on unsigned types

Fixes several remaining MSVC C4146 warnings where unary minus was applied to unsigned integers.
This should be the last PR containing MSVC C4146 warning fixes. I will remove the disablement of the warning as an error in a subsequent PR once the other pending PRs are completed.

Changes:

• Replace -(unsigned_value) with ~unsigned_value + 1 for offset calculations
• Use -1LL instead of -1ULL where signed values are intended
• Fix alignment padding calculation to avoid unsigned negation

Files changed:

• CoverageMappingGen.cpp, Lexer.cpp, Rewriter.cpp: Use two's complement for safe unsigned negation in offset calculations
• ItaniumCXXABI.cpp: Use signed literal for ABI-compliant null member pointer (-1)
• ExprConstant.cpp: Replace -1ULL with ~0ULL for bitmasks
• CodeGenMapTable.cpp: Fix sentinel value generation

All changes are mathematically equivalent and preserve existing behavior while eliminating compiler warnings.

Addresses #7573