Add support for specifying separate C and C++ compiler paths and args

[Colengms]

Related to: microsoft/vscode-cpptools#9266

Opening this to track updating compilerPath, compilerArgs, and compilerFragments fields to allow separate values to be provided for C vs. C++.

[Colengms]

Technically, a custom configuration provider could manage multiple subprojects within the same workspace, use multiple different compilers to build sources within the same project, or even build some files in the project with different sets of args that influence the include paths returned from the compiler.

Perhaps instead of applying this change to these fields in WorkspaceBrowseConfiguration, specifically, it should instead have an array of compilerPath/compilerArgs pairs, to allow all such combinations to be queried for include paths.

[bobbrow]

Perhaps instead of applying this change to these fields in WorkspaceBrowseConfiguration, specifically, it should instead have an array of compilerPath/compilerArgs pairs, to allow all such combinations to be queried for include paths.

Let's not do this until/unless it becomes a scenario our customers explicitly ask for. My opinion is that what most developers need is a pointer to *any* implementation of the STL so that auto-complete will work for them. I don't expect them to need N copies of compiler-specific implementations.