Modify clangService.ts for multiple sources support

[nokotan]

Associated Issue: #365 #298

Here's the contributing guide at https://github.com/wasdk/WebAssemblyStudio/wiki/Contributing

Summary of Changes

• change 1
  I have made changes to src/compilerServices/clangService.ts for the multiple files support.
  (almost same changes in Now We can Use Header File . #365; not updated for long time, I made this PR.)

Test Plan

Current test cases in tests/compilerServices/clangService.spec.ts are designed for single c source file and expected to rename this to "file.c", some test cases fail.
So I check my implementation manually... (should modify test cases, but I'm not sure that the modification is appropriate in this PR.)

Failing test cases:

• should compile C/Cpp -> Wasm
• should handle errors during compilation
• should throw an error when trying to compile more than one file

Screenshots/Videos (OPTIONAL)

[nokotan]

I've decided to rename branch, closing this issue.
Please refer #504.