Fad option for automatic debug output

[pow2clk]

Because the /Fd option requires a backslash on Windows to specify a directory, if this argument needs to be quoted, the closing quotation mark is escaped, which confuses the arguments provided.

The /Fad option explicitly specifies the directory to output the automatically named debug files to without the need for a trailing separator character.

The implementation reuses the significance of the DebugFile member, while adding a DebugDir member that indicates that DebugFile contains a directory name that needs to have both the platform-appropriate separator and the generated debug file name appended to it.

Fixes #2427

[pow2clk]

Tagging @otanter-at-ubi to make sure he finds this new PR

[AppVeyorBot]

:white_check_mark: Build DirectXShaderCompiler 1.0.2643 completed (commit https://github.com/Microsoft/DirectXShaderCompiler/commit/0548613c78 by @pow2clk)

[pow2clk]

Dusting this off after it came up again in triage.

[AppVeyorBot]

:white_check_mark: Build DirectXShaderCompiler 1.0.3525 completed (commit https://github.com/Microsoft/DirectXShaderCompiler/commit/82434db323 by @pow2clk)