`sizeof` may give incorrect result in editor for structs containing bitfields

Bug type: Language Service

Describe the bug

• OS and Version: Ubuntu 20.04
• VS Code Version: 1.61.0
• C/C++ Extension Version: v1.7.0-insiders2
• Other extensions you installed (and if the issue persists after disabling them): N/A
• If using SSH remote, specify OS of remote machine: N/A
• A clear and concise description of what the bug is, including information about the workspace (i.e. is the workspace a single project or multiple projects, size of the project, etc).

Steps to reproduce

Code sample 1

#include <iostream>

struct test1 {
    unsigned int a : 4;
    unsigned int b : 4;
    unsigned char c;
    unsigned char d;
};

int main() {
    std::cout << sizeof(test1) << std::endl;
}

Expected behavior

• sizeof(test1) gives 4 when the program runs. (when memory alignment is set to 4 bytes)
• However in the editor it shows 8.

Code sample 2

This time I add #pragma pack(1).

#include <iostream>

#pragma pack(1)
struct test1 {
    unsigned int a : 4;
    unsigned int b : 4;
    unsigned char c;
    unsigned char d;
};

int main() {
    std::cout << sizeof(test1) << std::endl;
}

Expected behavior

• sizeof(test1) gives 3 when the program runs. (when memory alignment is set to 1 byte)
• However in the editor it shows 6.

Start by reproducing the issue in the C/C++ extension with the two struct samples provided, comparing the editor's sizeof result with the program output. Trace the language-service handling of sizeof for bitfields, alignment, and #pragma pack. Done means the editor reports 4 and 3 for the two examples under the stated packing settings.