Collect warnings/errors into "validation log", expose through libclv API, print in driver

Previously, any warnings/errors were emitted directly to stderr by Clang and our WebCLReporter. Now these are collected in a structured form by a new WebCLDiag component and exposed through the API. The CLI validator uses the API to print out the log to stderr in a pretty similar format so existing tests which check for certain errors on invalid source etc continue to work.

Messages are currently quads with:
- severity (enum: note/warning/error)
- text (as formatted by Clang)
- full source code that caused the error (same as input source for preprocessing errors, transformed source otherwise)
- offset/length of the part of the source relevant to the error

This is exposed through the following API:

```
// Get number of validation log messages (notes, warnings, errors)
CLV_API cl_int CLV_CALL clvGetProgramLogMessageCount(
    clv_program program);

// Severity level of a validation log message
typedef enum {
    // Informative note
    CLV_LOG_MESSAGE_NOTE,
    // Warning
    CLV_LOG_MESSAGE_WARNING,
    // Error
    CLV_LOG_MESSAGE_ERROR
} clv_program_log_level;

// Get severity level of a validation log message
CLV_API clv_program_log_level CLV_CALL clvGetProgramLogMessageLevel(
    clv_program program,
    cl_uint n);

// Get text of a validation log message
CLV_API cl_int CLV_CALL clvGetProgramLogMessageText(
    clv_program program,
    cl_uint n,
    size_t buf_size,
    char *buf,
    size_t *size_ret);

// Determine if the given validation log message has associated source code
CLV_API cl_bool CLV_CALL clvProgramLogMessageHasSource(
    clv_program program,
    cl_uint n);

// Get the start offset of the relevant part of the source code
CLV_API cl_long CLV_CALL clvGetProgramLogMessageSourceOffset(
    clv_program program,
    cl_uint n);

// Get the length of relevant part of the source code
CLV_API size_t CLV_CALL clvGetProgramLogMessageSourceLen(
    clv_program program,
    cl_uint n);

// Get (a substring of) the source code associated with the log message
CLV_API cl_int CLV_CALL clvGetProgramLogMessageSourceText(
    clv_program program,
    cl_uint n,
    cl_long offset, // 0 for full source
    size_t len, // size_t(-1) for full source
    size_t buf_size,
    char *buf,
    size_t *size_ret);
```

Unfortunately not all code in Clang uses the diagnostics system, so e.g. the error/warning counts are still always emitted to stderr. This would need a fix in clang.

Still missing is the location in the original input source that caused the diagnostic. The line numbers are currently lost in our rewrite/reparsing passes. This could be fixed by using Clang preprocessor #line directives suitably, at which point original source line numbers could be added to the API for better graphical debugging etc.