Add UI to provide args for templates, to enable template IntelliSense

* Operating System and version

MacOS 10.13.4

* VS Code version and if you are using the Insiders build

VSCode 1.24.0-Insiders #2404210629c744e6237a14d7b5fa852e24c6e898

* C/C++ extension version

0.16.1

* step-by-step instructions to reproduce the issue

Create at least one class declaration in a header file.
Use a template on this class
Intentionally create a syntax error
Note that no error squiggles are generated and no error or warning given in the Problems pane.
In the example below, I am showing quite a few compiler errors (there are plenty in this file). One in particular is the errant use of override. This shows up in the compiler output, but not in the error squiggles or in the Problems pane:

* A small code sample, zipped up project, or open source repo we can use to verify the bug

Errant header file attached
LinkedList.h.zip

* Any log messages present in the Output window (use "C_Cpp.loggingLevel": "Information" in settings.json)

The logs indicate that it thinks it is looking for syntax errors:
IntelliSense Engine = Default.
The extension will use the Tag Parser for IntelliSense when #includes don't resolve.
Autocomplete is enabled.
Error squiggles are enabled.
File exclude: **/.git
File exclude: **/.svn
File exclude: **/.hg
File exclude: **/CVS
File exclude: **/.DS_Store
File exclude: **/.o
File exclude: **/.d
File exclude: **/.vscode
Search exclude: **/node_modules
Search exclude: **/bower_components
Search exclude: **/.vscode
Code browsing service initialized
Folder: /usr/local/include/ will be indexed
Folder: /usr/local/Cellar/gcc/7.3.0_1/include/c++/7.3.0/ will be indexed
Folder: /Users/dustinlane/Desktop/VSCode_Test/ will be indexed
Folder: /System/Library/Frameworks/ will be indexed
Folder: /Library/Frameworks/ will be indexed
Discovering files...
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/LinkedList.h
Processing folder (recursive): /usr/local/include/
Processing folder (recursive): /usr/local/Cellar/gcc/7.3.0_1/include/c++/7.3.0/
Processing folder (recursive): /Users/dustinlane/Desktop/VSCode_Test/
Processing folder (recursive): /System/Library/Frameworks/
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/LinkedList.h
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/LinkedList.h
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/LinkedList.cpp
Shutting down IntelliSense server: /Users/dustinlane/Desktop/VSCode_Test/C++_Test/main.cpp
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/LinkedList.h
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/LinkedList.h
Processing folder (recursive): /Library/Frameworks/
Discovering files: 301712 file(s) processed
0 file(s) removed from database
Done discovering files.
Parsing open files...
Parsing remaining files...
Parsing: 0 files(s) processed
Done parsing remaining files.
Done parsing open files.
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/main.cpp
Checking for syntax errors: file:///Users/dustinlane/Desktop/VSCode_Test/C%2B%2B_Test/LinkedList.h

Begin with the attached LinkedList.h.zip and the reproduction steps, then inspect the extension's IntelliSense and UI entry points for template argument configuration. Done means users can provide template arguments through the UI and template IntelliSense handles them; also verify the reported diagnostics scenario.