Using the half type does not generate the Float16 capability

Found while reviewing https://github.com/KhronosGroup/SPIRV-Tools/pull/6009.

Surprisingly, using the `half` type does not generate SPIR-V modules that declare the **Float16** capability. We have some tests that are checking that the right capability is declared, for example:

https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/4eea290c449fca2efd28cfa46d3946c5feaf988c/test/half_extension.ll#L17

However, this is unfortunately matching `Capability Float16Buffer`, so it is not actually checking for `Capability Float16`. If I change this check instead to:

```
; CHECK-SPIRV: Capability {{Float16 *$}}
```

Then the test starts to fail.

Recommended actions:

1. Change all tests to check for `Float16` at the end of the line, similar to the check above.
* Are there any other capabilities that are the prefix of another capability that may be affected similarly?
2. Figure out why **Float16Buffer** is being declared and not **Float16**. Possibly something is going wrong here?

https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/4eea290c449fca2efd28cfa46d3946c5feaf988c/lib/SPIRV/libSPIRV/SPIRVType.h#L214-L224

Start with test/half_extension.ll and update the capability checks so Float16 is matched at the end of the line; review related capability checks for the same prefix issue. Then inspect lib/SPIRV/libSPIRV/SPIRVType.h#L214-L224 to trace why using half declares Float16Buffer instead of Float16, and run the affected tests to confirm the capability output.

Written by the indexing model from the issue text.